Dissertations / Theses on the topic 'Intermolecular Bonding'

Advanced analytical techniques were utilized to investigate the intermolecular forces in several nanostructured materials. Techniques including, but not limited to, isothermal titration calorimetry (ITC), variable temperature Fourier transform infrared (FTIR) spectroscopy, and ultraviolet-visible (UV-Vis) thermal curves were used to study the fundamental interactions present in various nanomaterials, and to further probe the influence of these interactions on the overall behavior of the material. The areas of focus included self-assembly of surfactant micelles, polycation complexation of DNA, and temperature-dependent hydrogen bonding in polymeric systems. ITC was successfully used to determine the low critical micelle concentration (CMC) for a novel gemini surfactant with limited water solubility. CMCs were measured at decreasing methanol molar fractions (xMeOH) in water and the resulting linear relationship between CMC and methanol concentration was used to mathematically extrapolate to a predicted CMC at xMeOH = 0. Using this technique, the CMC value for the novel gemini surfactant was predicted to be 0.037 ± 0.004 mM. This extrapolation technique was also validated with surfactant standards. ITC was also used to investigate the binding thermodynamics of polyplex formation with polycations and DNA. The imidazolium-containing and trehalose-based polycations were both found to have endothermic, entropically driven binding with DNA, while the adenine-containing polycation exhibited exothermic DNA binding. In addition, ITC was also used to confirm the stoichiometric binding ratio of linear polyethylenimine and DNA polyplexes as determined by a novel NMR method. Dynamic light scattering (DLS) and zeta potential measurements were also performed to determine the size and surface charge of polyplexes. Circular dichroism (CD) and FTIR spectroscopies provided information regarding the structural changes that may occur in the DNA upon complexation with polymers. UV-Vis thermal curves indicated that polyplexes exhibit a greater thermal stability than DNA by itself. Variable temperature FTIR spectroscopy was used to quantitatively compare the hydrogen bonding behavior of multi-walled carbon nanotube (MWCNT)-polyurethane composites. Spectra were collected from 35 to 185 deg C for samples containing various weight percent loadings of MWCNTs with different hydrogen bonding surface functionalities. Peak fitting analysis was performed in the carbonyl-stretching region for each sample, and the hydrogen-bonding index (Rindex) was reported. Rindex values were used to quantitatively compare all of the composite samples in regards to temperature effects, weight percent loadings of MWCNTs, and the different functionalizations. In general, higher weight percent loadings of the MWCNTs resulted in greater Rindex values and increased hydrogen bond dissociation temperatures. In addition, at 5 and 10 wt% loadings the initial Rindex values displayed a trend that tracked well with the increasing hydrogen bonding capacity of the various surface functionalities.
Ph. D.

In this work three different approaches to the study of intermolecular interactions are shown. The aim is to further the understanding of specific intermolecular interactions with a view to eventually allowing the prediction and design of crystal structures from the initial molecular building blocks: crystal engineering. All three approaches make use of crystal structural information derived from X-ray and/or neutron diffraction studies. The three approaches are: • Data base approach. Specifically, the study of occurrence of hi- and tri-furcated hydrogen bonds in the Crystallographic Structural Database, and the analysis of the frequency with which they occur and the geometric restrictions of such interactions.• Analysis of a series of compounds, where there are small changes in the molecular structure as the series progresses. The influence of these changes in the molecular structure on the crystal structure is considered. The series studied was the 4-amino- 4'-hydroxydiphenylalkanes as well as some of the corresponding 4-amino-4'- hydroxydiphenylsulphides and -alkylsulphides.• Detailed analysis of individual structures to identify the intermolecular interactions that are influencing the structure. The compounds analysed in this part were 2,4,6- m.s-(4-chlorophenoxy)-l,3,5-triazene co-crystallised with tribromobenzene, triphenylisocyanurate co-crystallised with trinitrobenzene, 4,4'-dinitrotetrapheny 1 methane, 2,3-dichloro-l,4-diethynyl-l,4-dihydroxy-napthalene and, 4,4-diphenyl- 2,5-cyclohexadienone.

The implications of chain topology and intermolecular interactions on the electrospinning process were investigated for linear and randomly branched polymers. Empirical correlations were developed based on solution rheological measurements that predict the onset of electrospun fiber formation and average fiber diameter. In particular, for neutral, non-associating polymer solutions, the minimum concentration required for fiber formation was the entanglement concentration (Ce), and uniform, bead-free fibers were formed at 2 to 2.5 Ce. This was attributed to entanglement couplings stabilizing the electrospinning jet and preventing the Raleigh instability. Moreover, the influence of molar mass and degree of branching on electrospun fiber diameter was eliminated when the polymer concentration was normalized with Ce, and the fiber diameter universally scaled with C/Ce to the 2.7 power. Polymers modified with quadruple hydrogen bonding groups were investigated to determine the role of intermolecular interactions on the solution rheological behavior and the electrospinning process. In nonpolar solvents, the hydrogen bonding functionalized polymers displayed significant deviation from the electrospinning behavior for neutral solutions due to the strong intermolecular associations of the multiple hydrogen bonding groups. The predicted electrospinning behavior was recovered when the hydrogen bonding interactions were screened with a polar solvent. Moreover, it was observed that branching and multiple hydrogen bonding afforded significant processing advantages compared to functionalized, linear analogs of equal molar mass. For example, branched chains in the unassociated state possessed a larger Ce compared to the linear chains, which indicated a lower entanglement density of the former. However, in the associated state the linear and branched chains possessed nearly equivalent Ce values, suggesting a similar entanglement density. Thus, the branched polymers displayed significantly lower viscosities in the unassociated state compared to linear polymers, while still retaining sufficient entanglements in the associated state due to the reversible network structure of the multiple hydrogen bond sites. The solution rheological and processing behavior of polyelectrolyte solutions was also investigated to discern the role of electrostatic interactions on electrospun fiber formation. In particular, the polyelectrolyte solutions formed nano-scale electrospun fibers with an average fiber diameter 2 to 3 orders of magnitude smaller than neutral polymer solutions of equivalent viscosity and C/Ce. This was attributed to the very high electrical conductivity of the polyelectrolyte solutions, which imparted a high degree of charge repulsion in the electrospinning jet and increased the extent of plastic stretching in the polymer filament. In fact, the average diameter of the polyelectrolyte fibers under certain conditions was less than 100 nm, which makes them good candidates for protective clothing applications due to their high specific surface area. Moreover, the neutral polymer solution electrospinning behavior was recovered after the addition of NaCl, which screened the electrostatic charge repulsions along the polyelectrolyte main chain. Finally, electrospun, biocompatible phospholipid membranes were produced from solutions of entangled worm-like lecithin micelles. This is the first example of successfully electrospinning low molar mass, amphiphilic compounds into uniform fibers. Electrospinning the phospholipid worm-like micelles into nonwoven fibrous mats will afford direct engineering of bio-functional, high surface area membranes without the use of multiple synthetic steps, complicated electrospinning setups, or post processing surface treatments.
Ph. D.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2009.
ENGLISH ABSTRACT: This thesis reports an in-depth investigation of the intermolecular interaction motifs in secondary, primary and ammonium carboxylate salts. The investigation was conducted using the Cambridge Structural Database (CSD), together with a systematic steric-specific experimental study. The tendency in the literature has been to analyse organic salt crystal structures in terms of hydrogen bonding patterns, almost ignoring cation-anion interactions. This study focuses on the cation-anion interactions in secondary, primary and ammonium carboxylate salts, which have a direct effect on the formation of specific structural motifs. The ideas of ring-stacking and ring-laddering, which arise from the tendency of cations and anions to arrange themselves so as to maximise electrostatic interactions, have been applied to ammonium carboxylate salts. An extensive survey of organic ammonium carboxylate salt structures in the CSD has been carried out. The structural motifs in ammonium carboxylates were investigated, and a set of predictive rules for the pattern of intermolecular interactions in these salts was developed. Using these results, the formation of ring-stacking or ring-laddering in primary ammonium carboxylate salts can be predicted. The results from the CSD survey are discussed in Chapter 3. An experimental study has been carried out, which complements the results obtained from the CSD survey. The experimental study formed 19 novel ammonium carboxylate salts, of which 2 formed hydrates and 2 co-crystals of salts. The experimental results confirm what was found in the CSD survey, and this is discussed in Chapter 4. This study has found that the principle of ring-stacking and ring-laddering can be applied in a general form to the crystal structures of organic ammonium carboxylate salts. The size of the cation and the anion in these salts has a significant effect on the formation of structural motifs in the solid state. Interactions between cation and anion substituents also play an important role in the formation of particular structural motifs in ammonium carboxylate salts.
AFRIKAANSE OPSOMMING: In hierdie tesis word die intermolekulêre interaksie motiewe in die sekondêre, primêre en ammonium karbosilaat soute in-diepte ondersoek. Die studie is gedoen met behulp van die Cambridge Strukturele Databasis (CSD), saam met ‟n sistematiese steriesspesifieke eksperimentele studie. Die neiging in die literatuur is om organiese sout kristal strukture in terme van waterstofbindings patrone te analiseer sonder om katioon-anioon interaksies in ag te neem. Die studie fokus juis op hierdie katioon-anioon interaksies tussen sekondêre, primêre en ammonium karbosilaat soute wat ‟n direkte effek het op die vorming van spesifieke strukturele motiewe naamlik „ring-stacking‟ en „ring-laddering‟ wat hul oorsprong kry vanaf die neiging van katione en anione om hulself op so ‟n wyse te rangskik sodat die elektrostatiese interaksies ‟n maksimum kan bereik, op die ammonium karboksilaat soute. ‟n Volledige ondersoek van ammonium karboksilaat soute in die CSD is gedoen. Die strukturele motiewe in ammonium karboksilaat is ondersoek, en ‟n stel reels wat die patrone van intermolekulêre interaksies in hierdie soute voorspelis ontwikkel. Hierdie resultate kan gebruik word om die vorming van „ring-stacking‟ en „ring-laddering‟ in primêre ammonium karbosilaat soute te voorspel. Die resultate van die CSD ondersoek word bespreek in Hoofstuk 3. ‟n Eksperimentele studie is uitgevoer en die resultate hiervan komplimenteer die resultate van die CSD ondersoek. In die eksperimentele studie is 19 nuwe ammonium karboksilaat soute gekristaliseer, waarvan 2 hidraat-soute en 2 ko-kristal-van-soute is. Die eksperimentele resultate bevestig die bevindings van die CSD ondersoek, en dit word bespreek in Hoofstuk 4. Hierdie studie het gevind dat die beginsel van „ring-stacking‟ en „ring-laddering‟ kan in „n algemene vorm in die kristal strukture van organiese ammonium karboksilaat soute toegepas word. Die grootte van die katioon en anion in hierdie soute het ‟n beduidende effek op die vorming van strukturele motiewe in die vaste toestand. Interaksie tussen die katioon en anioon substituente speel „n belangrike rol in die vorming van spesifieke motiewe in ammonium karbosilaat soute.

A blue shift in the CH stretching vibration of formic acid cyclic dimer of 6.6 ern" (symmetric) and 3 ern" (antisymmetric) is observed by high resolution Raman (symmetric) and infrared (antisymmetric) spectroscopy. This is corroborated by theoretical ab initio calculations where blue shifts in the CH stretching vibration of 12.79 ern" (symmetric) and 10.26 ern" (antisymmetric) are calculated (CP corrected MP2/6-311++G(d,p) level of theory). This is unusual due to the CH bond not playing a direct part in the bonding of the dimer. The electric dipole moment derivative curve with respect to bond length of the CH bond in formic acid is found to be unusual. The equilibrium bond length is on the negative gradient side of the maximum of the dipole, and this has been used to explain interesting behaviour observed, including the blue shift of the CH stretching vibration and how the contribution of electrostatics to the interaction energy can cause a blue shift of the stretching vibration in the spectrum. A mechanism is proposed where the electron density is transferred from the CH bond, through to the OH site where bonding does take place. This in turn causes the CH bond to have increased polarity, and therefore the bond contracts due to this interaction. Several chloroform complexes are investigated, which show either blue shifting or red shifting of the CH stretching vibration. Complexation with dimethyl ether shows an experimental red shift of the CH stretching vibration of chloroform of -1.5 ern", and a theoretical shift of -2.11 crn'. The complex of chloroform with trimethyl amine shows an experimental red shift of the CH stretching vibration of chloroform of -54 ern", and a theoretical prediction of -79.51 ern". Both of these complexes show a 1: 1 stoichiometric equation. The chloroform self dimer shows blue shifts in the CH stretching vibration, calculated to be 2.1 and 8.8 ern", experimental results are currently inconclusive. Morokuma Kitaura energy decomposition has been used to understand the energy contributions to intermolecular bonding. Electrostatic interaction and exchange repulsion have been shown to be the main contributions to bonding, but some unusual cases, for example the CH bond of formic acid cyclic dimer, have shown electrostatics to cause a blue shift. - 3 - A tuneable stimulated Raman photoacoustic spectroscopy (PARS) set up has been further applied to the trace detection of H2' and has achieved a detection limit of 6.69 ppm by volume. A non-dispersive Raman shifter method has also been investigated as a simpler alternative to the tuneable PARS set up and has achieved a less sensitive detection limit of 108 ppm by volume. Methane has also been detected qualitatively via this method, using the Raman shifter as a source of infrared light.

This thesis presents different studies on hydrogen bonds that occur between two molecules. It is because of the biological importance of DNA that the base pairs, and other compounds with similar hydrogen bonds as these base pairs, have been studied in this thesis. The main objective of the thesis is to understand Resonance Assisted Hydrogen Bonds (RAHB). There are two well-defined parts in this thesis. First, electron delocalization indices are used in order to have a better image of the HB formation in our systems. Secondly, a decomposition of the hydrogen bond energy allows understanding the components of the bonding energy and identifying which are responsible for the extra stabilization that compounds with RAHB show. Both computational approaches show that the π electrons are not responsible for the enhanced stability of the RAHB
Aquesta tesi presenta diferents estudis sobre enllaços d’hidrogen que tenen lloc entre dues molècules. La importància biològica de l’ADN ha portat a escollir com a sistemes d’estudi les seves parelles de bases i altres compostos que mostren enllaços semblants. El principal objectiu de la tesi és entendre els enllaços d’hidrogen assistits per ressonància (RAHB). En aquesta tesi hi ha dues parts ben diferenciades. Primerament, s’usen índexs de deslocalització electrònica, que permeten tenir una imatge més clara de la formació dels enllaços d’hidrogen en els nostres sistemes. En segon lloc, la descomposició de l’energia dels enllaços d’hidrogen permet entendre els components de l’energia d’enllaç i identificar els responsables de l’extra estabilització que presenten els compostos amb RAHB. Els dos enfocaments computacionals mostren que els electrons π no són responsables de la millor estabilitat dels RAHB

Doctor of Philosophy
Department of Chemistry
Christer B. Aakeroy
A family of three 2-aminopyrazine derivatives were prepared and co-crystallized with thirty carboxylic acids. Our theoretical charge calculations and experimental results from 90 reactions demonstrated that decreasing the charge on the hydrogen-bond acceptor sites results in a decrease of the supramolecular yield (the frequency of occurrence of the desired outcome). However, synthon crossover (undesired connectivity) was observed 7/12 times and was unavoidable due to competitive binding sites present in the N-heterocyclic bases chosen. To avoid synthon crossover, we used a strategy based on geometric bias. We utilized hydrogen-bonding two-point contacts and halogen-bonding single-point contacts for supramolecular reactions with the 2-aminopyrazine family of compounds. The desired two-point contact and single-point contact (N•••I or N•••Br) appeared in 9/9 times even in the presence of other potentially interfering intermolecular interactions. In addition, the role of charge in controlling the presence/absence of proton transfer was also highlighted. To establish a hierarchy in halogen-bonding interactions we designed and synthesized a library of eight molecules equipped with two different halogen bond donors and combined with variety of halogen-bond acceptors. 11 Halogen-bonded co-crystals were obtained; however, positional disorder of I/Br atoms obscures a complete analysis. This problem was solved by introducing asymmetry in the halogen-bond donor molecules. Finally, successfully demonstrated an unprecedented hierarchy in halogen-bond interactions based on electrostatics. We developed high-yielding Suzuki-Miyaura coupling reactions of tetraboronic pinacolyl ester cavitand to iodoarenes with a range of functional groups (electron withdrawing/donating group and a heterocycle) that show robustness and versatility, making it a ‘launch pad’ for the synthesis of many new cavitands in a facile manner. We have also successfully demonstrated cavitand functionalization from tetraaldehyde to tetraoximes using ‘solvent assisted grinding’, irrespective of the position of the aldehyde. Finally, we prepared tetra-substituted pyridyl and carboxylic acid cavitands having an ellipsoidal cavity capable of encapsulating asymmetric guest molecules and was subsequently obtained the first of its kind, C[subscript]2v symmetric molecular capsule with encapsulated asymmetric guest molecule.

This project investigated the effects of co-crystallisation on hierarchical assembly and solid-state stability of metallosupramolecular cages of Fe(II), Co(II) and Co(III). The prospects for encapsulating and/or connecting the metallosupramolecular cages with halogen bonded intermolecular networks were explored. A wide range of analyses were performed, both in solution and in the solid state, to assess the chemical and structural diversity resulting from the inclusion of halogen bonding components within crystals of these novel metal complexes.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2009.
Co-crystals are excellent materials for studying intermolecular interactions in the solid-state and can be used to further our knowledge of the balance between strong and weak intermolecular interactions. The O–H∙∙∙∙∙∙Narom synthon was chosen as the focus of this investigation of hydrogen bonding motifs. The starting materials selected all have two hydrogen bond donor and/or acceptor sites for the formation of extended networks. All molecules are also aromatic such that the influence of weaker π∙∙∙∙∙∙π interactions can be included in the study. Two 3x3 grids of related co-crystals were produced from these starting materials and are reported in this thesis as part of an ongoing investigation into a broader set of co-crystals. A part of the work describes the investigation of co-crystals prepared by the combination of related benzenediol and diazine isomers taken from a 3x3 grid. The solid-state structures of each of the six starting materials are discussed briefly to describe the nature of intermolecular interactions involved in the single component crystals. Trends in hydrogen-bonding patterns as well as the weaker interactions identified in the starting materials, can be used to recognise those in the subsequent multi-component crystals. Thirteen co-crystal compounds were obtained, of which twelve structures are novel. Each of these co-crystal structures is discussed in terms of intermolecular interactions and packing in the solid state. Hydrogen-bonding patterns and structural similarities are highlighted in related co-crystal structures as well as between co-crystals and their respective starting materials. The combination of benzenediol isomers with benzodiazine isomers yielded seven novel co-crystal structures in a second 33 grid is reported. The structure of phthalazine, which has not yet been reported, is included in addition to these co-crystals, while the structures of quinazoline and quinoxaline that were retrieved from the CSD are discussed briefly. Co-crystal structures are discussed individually, focusing on the intermolecular interactions that are significant to the structural architecture of the compound. Certain co-crystals that display structural similarities with structures of the 3x3 grid, as well as with co-crystals presented in Chapter 3, are discussed in the relevant sections. Lastly, two extended pyridyl diyne ligands that were synthesised for use in future co-crystallisation studies similar to those reported earlier are briefly highlighted. The crystal structures of the pure compounds and of a hydrate of one of the ligands were obtained and discussed briefly. To date only one of these structures has been reported in the literature.

碩士
國防大學中正理工學院
應用化學研究所
91
In this article we studied the amines (N(CH3)nH3-n) combined with different kinds of molecule to create the intermolecular hydrogen bonding and inter-ionic hydrogen bonding complex, from that way, discussing the influence of molecular structure stability by two kinds of hydrogen bonding systems. In the intermolecular hydrogen bonding system, we chosen amines combined with nitric acid (HNO3) or hydrogen halide (HX,X=F, Cl or Br) complex, to compare and analyze the affect of hydrogen bonding by methyl group and steric effect. However, in the part of ionic-inter hydrogen bonding, selected the amines combined with hydrogen halide complex for researching object, to discuss the distinction of intermolecular hydrogen bonding and inter-ionic hydrogen bonding. As the result of calculation, we found that methyl group really could strength the stability at intermolecular hydrogen bonding. Along with the addition of temperature because of steric effect and entropy, the stability sequence of Gibbs free energy (△G) would have different variations. In the amines combined with hydrogen halide complex, when methyl group added gradually and atomic number of halogen atom became augmented (Cl or Br), contrasted the migration of hydrogen atom position in hydrogen bonding system, we could find the tendency of total system transferred from intermolecular hydrogen bonding to inter-ionic hydrogen bonding. In the case of inter-ionic hydrogen bonding system, its binding energy between ions including both the coulombic attraction energy and hydrogen bonding energy. In this system, the overall energy between this ionic pairs is decreased whenever the number of CH3 is increasing due to proton accept or electron donor effect of methyl group.

The thesis entitled “Unravelling the Nature of Halogen and Chalcogen Intermolecular Interactions by Charge Density Analysis" consists of five chapters. A basic introductory section describes the topics relevant to the work and the methods and techniques utilized. The main focus of the present work is to characterize the interaction patterns devoid of strong classical hydrogen bonds. The case studies include halogen bonds and hydrogen bonds involving bromine (as a halogen bond donor and hydrogen bond acceptor), intermolecular chalcogen bond formation involving sulphur, type I Br Br contacts, type II F F and F S interactions and S-H S hydrogen bonds. Chapter 1 discusses experimental and theoretical charge density analyses on 2,2-dibromo-2,3-dihydroinden-1-one which has been carried out to quantify the topological features of a short C Br···O halogen bond with nearly linear geometry (2.922Å, C Br···O=172.7) and to assess the strength of the interactions using the topological features of the electron density. The electrostatic potential map indicates the presence of the “- hole” on bromine while the interaction energy is comparable to that of a moderate O-H O hydrogen bond. In addition, the energetic contribution of C-H···Br interaction is demonstrated to be on par with that of the C-Br···O halogen bond in stabilizing the crystal structure. Chapter 2 discusses an organic solid, 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole that has been designed to serve as an illustrative example to quantitatively evaluate the relative merits of halogen and chalcogen bonding in terms of charge density features. The compound displays two polymorphic modifications, one crystall zing in a non-centrosymmetric space group (Z =1) and the other in a centrosymmetric space group with two molecules in the asymmetric unit (=2). Topological analysis based on QTAIM clearly brings out the dominance of chalcogen bond over the halogen bond along with an indication that halogen bonds are more directional compared to chalcogen bonds. The cohesive energies calculated with the absence of both strong and weak hydrogen bonds as well as stacking interaction are indicative of the stabilities associated with the polymorphic forms. Chapter 3 discusses the role of a type I C-Br Br-C contact and what drives the contact i.e. how a dispersive interaction is stabilized by the remaining contacts in the structure. In the process we observe the role the Br2Cl motif which is quite unique in its nature. Also the role of the bromine atoms in stabilizing the stacking interactions has been shown by the electrostatic potentials which are oriented perpendicular to the plane of the benzene ring. Chapter 4 discusses the enigmatic type II C-F F-C and C-FS-C interactions in pentafluorophenyl 2,2- bithiazole. Both the interactions are shown to be realistic “-hole” interactions based on high resolution X-ray charge density analysis. As fluorine is the most electronegative element, its participation in halogen bonding wherein the electrostatic potential around the atom gets redistributed to form regions of electron depletion and accumulation had time and again been speculated but never observed. In this chapter the experimental charge dnsity analysis clearly identifies the “-hole” on fluorine and distinguishes the C-F S-C interaction as a halogen bond rather than the chalcogen bond. Chapter 5 discusses the experimental charge density analysis of the hitherto unexplored S-H S hydrogen bond in crystal structures. The work highlights how relatively small is the number of crystal structures which are constructed by the S-H S hydrogen bond compared to the X-H S hydrogen bond via Cambridge Structural Database (CSD) analysis. The potential S-H S hydrogen bond is studied in three isomeric mercaptobenzoic acids with experimental charge density collected on 2-mercaptobenzoic acid and theoretical estimates made on 3- and 4-mercaptobenzoic acid. The strength and directionality of the S-H S hydrogen bond is demonstrated to be mainly due to the conformation locking potential of intramolecular S O halogen bond.

The thesis entitled “Unravelling the Nature of Halogen and Chalcogen Intermolecular Interactions by Charge Density Analysis" consists of five chapters. A basic introductory section describes the topics relevant to the work and the methods and techniques utilized. The main focus of the present work is to characterize the interaction patterns devoid of strong classical hydrogen bonds. The case studies include halogen bonds and hydrogen bonds involving bromine (as a halogen bond donor and hydrogen bond acceptor), intermolecular chalcogen bond formation involving sulphur, type I Br Br contacts, type II F F and F S interactions and S-H S hydrogen bonds. Chapter 1 discusses experimental and theoretical charge density analyses on 2,2-dibromo-2,3-dihydroinden-1-one which has been carried out to quantify the topological features of a short C Br···O halogen bond with nearly linear geometry (2.922Å, C Br···O=172.7) and to assess the strength of the interactions using the topological features of the electron density. The electrostatic potential map indicates the presence of the “- hole” on bromine while the interaction energy is comparable to that of a moderate O-H O hydrogen bond. In addition, the energetic contribution of C-H···Br interaction is demonstrated to be on par with that of the C-Br···O halogen bond in stabilizing the crystal structure. Chapter 2 discusses an organic solid, 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole that has been designed to serve as an illustrative example to quantitatively evaluate the relative merits of halogen and chalcogen bonding in terms of charge density features. The compound displays two polymorphic modifications, one crystall zing in a non-centrosymmetric space group (Z =1) and the other in a centrosymmetric space group with two molecules in the asymmetric unit (=2). Topological analysis based on QTAIM clearly brings out the dominance of chalcogen bond over the halogen bond along with an indication that halogen bonds are more directional compared to chalcogen bonds. The cohesive energies calculated with the absence of both strong and weak hydrogen bonds as well as stacking interaction are indicative of the stabilities associated with the polymorphic forms. Chapter 3 discusses the role of a type I C-Br Br-C contact and what drives the contact i.e. how a dispersive interaction is stabilized by the remaining contacts in the structure. In the process we observe the role the Br2Cl motif which is quite unique in its nature. Also the role of the bromine atoms in stabilizing the stacking interactions has been shown by the electrostatic potentials which are oriented perpendicular to the plane of the benzene ring. Chapter 4 discusses the enigmatic type II C-F F-C and C-FS-C interactions in pentafluorophenyl 2,2- bithiazole. Both the interactions are shown to be realistic “-hole” interactions based on high resolution X-ray charge density analysis. As fluorine is the most electronegative element, its participation in halogen bonding wherein the electrostatic potential around the atom gets redistributed to form regions of electron depletion and accumulation had time and again been speculated but never observed. In this chapter the experimental charge dnsity analysis clearly identifies the “-hole” on fluorine and distinguishes the C-F S-C interaction as a halogen bond rather than the chalcogen bond. Chapter 5 discusses the experimental charge density analysis of the hitherto unexplored S-H S hydrogen bond in crystal structures. The work highlights how relatively small is the number of crystal structures which are constructed by the S-H S hydrogen bond compared to the X-H S hydrogen bond via Cambridge Structural Database (CSD) analysis. The potential S-H S hydrogen bond is studied in three isomeric mercaptobenzoic acids with experimental charge density collected on 2-mercaptobenzoic acid and theoretical estimates made on 3- and 4-mercaptobenzoic acid. The strength and directionality of the S-H S hydrogen bond is demonstrated to be mainly due to the conformation locking potential of intramolecular S O halogen bond.

博士
國立臺灣大學
化學研究所
82
In this dissertation, the intermolecular hydrogen bonding of hydrogen bonding of phenols were studied by means of theoretical calculations and IR-Matrix isolation technique. In part I, we mainly investigated the NH--Pi hydrogen-bonds of pyrrole multimers. The optimized structure of pyrrole dimer shows that the NH of the proton donor moiety points to the center of the C3-C4 bond of the proton acceptor moiety.The NH--Pi hydro-gen bonding energies of pyrrole dimer calculated by MP2/6-31G(d) and HF/6-31G(d) after BSSE and ZPVE corrections are -4.64 and -2.39 kcal/mol, respectively. By ccomparing the results of MP2/6-31G(d) and HF/6-31G(d) calculations,we found that the electron correlation energy contributes about a half of hydrogen bonding energy. In addition to dimer, the possible stable structures of trimers and tetramers were also calculated. As to the studying of pyrrole vibrational spectra, we completed the assignment of Matrix isolation-IR spectra of pyrrole monomer with the help of calculated absortion frequencies, intensities and displacements of each normal mode.Molecular complexes formed between pyrrole and other proton acceptors such as benzene, trriethylamine and pyridine, were investigated. In part II, we examined the internal rotation of the OH group along the C-O bond for substituted phenols and intramolecular hydrogen bonding of 2-fluorophenol and 2-chlorrophenol.We investi-gated the OH internal rotation of mono-substituted flourophenols and found that the order of molecular energy is 3-flourophenol >4-flourophenol > trans-2-flourophenol, but the order of barrier heights is reversed. In addition to flourophenols,the OH internal rotation of other mono-substituted phenols with -Cl, -NH2, -NO2,-CN, -CHO, -CF3, group were investigated.

Intermolecular interactions appear to be well understood in a broad sense today; at a deeper molecular level, it is still evolving. Spectroscopy in this isolated state proved to be a first step toward understanding the intermolecular interaction at the molecular level. Microwave spectroscopy offers precise structural information on the near-equilibrium geometry of small dimers and trimers in isolation. Computational studies like the Atoms in Molecules (AIM), non-covalent index plots (NCI), and natural bond orbital analysis (NBO) are used to augment rotational spectroscopic investigations. The Ka = 1 transitions of H2S dimer and several isotopomers were observed in a pulsed nozzle Fourier transform microwave spectrometer. These transitions give unequivocal proof that, at ultra-low temperatures, hydrogen sulfide forms S-H⸳⸳⸳S hydrogen-bonded dimer in the same way as water does, even though ice and solid H2S seem substantially different in bulk. Also, using the AIM theory, we have shown that H2S dimer satisfies the necessary and sufficient criterion proposed by Koch and Popelier to be hydrogen-bonded. Although we recently highlight the arbitrariness in relying on some computational tools to characterize a bond. The weakly bound trimer between two hydrogen sulfide molecules and one water molecule, (H2S)2H2O, was identified from its rotational spectrum. The break with axial molecular symmetry allowed us to investigate (H2S)2H2O at a level of structural detail that has not yet been possible for (H2O)3 and (H2S)3 with rotational spectroscopy owing to their zero-dipole moment. Analysis of experimental results reveals that the three monomers are bound in a triangular arrangement through S-H⸳⸳⸳S, O-H⸳⸳⸳S, and S-H⸳⸳⸳O hydrogen bonds with a fair amount of co-operativity. High-resolution spectroscopic data may be used to validate the correctness of a model intermolecular potential energy hyper-surface. In this regard, we have measured the donor-acceptor interchange tunnelling splitting in the ground vibrational state of Ar-(H2O)2. In the previous investigations, the donor-acceptor tunnelling splitting in fully deuterated species, Ar-(D2O)2, was measured to be 106 MHz. However, it could not be measured for the Ar-(H2O)2,as the splitting was expected to be several GHz. With the help of a fourfold periodic potential, we have accurately predicted the fingerprints of donor-acceptor interchange tunnelling transitions and measured the splitting of 4257.41(4) MHz in Ar-(H2O)2. Lastly, we have looked beyond hydrogen bonding and explored other intermolecular bonding across the Periodic Table. The slopes of the binding energy versus electron density at the bond critical point were derived for each main group element. Our results show that intermolecular bonding can be classified into two types: intermolecular bonding (IMB) with a shared shell molecule (IMB-S) and intermolecular bonding (IMB) with a closed shell molecule (IMB-C). The IMB-S includes hydrogen, halogen, chalcogen, pnictogen, tetrel (excluding carbon bonds), and boron bonds (but not triel bonds). IMB-C contains lithium, sodium, beryllium, magnesium, triel (excluding boron bonds) and carbon bonds. The binding energy versus electron density plot of the IMB-S class generally has a low slope, whereas the IMB-C type has a high slope. Carbon bonds are distinct from the other members of the group. Carbon is a hesitant partner in tetrel bonds due to the absence of lower energy d-orbitals. The electron density between the two atoms is extremely low, and the binding energy grows fast with electron density, resulting in a high slope value for the carbon bond. The slopes for the Li, Na, Be, Mg, Ca-bonds were found out to be comparable, whereas the slope for the hydrogen bond remains standout. Several similarities eventually lead us to propose a common name, ‘Alkalene bond,’ for the intermolecular bonding in alkali and alkaline earth metals.

The nature of interactions within a molecule, i.e. chemical bonding, is well understood today. However, our understanding about intermolecular interactions, which has great relevance in nature, is still evolving. Historically there are two types of intermolecular interactions, van der Waals interaction and hydrogen bonding. However, there has been an upsurge of interest in the halogen bonding and lithium bonding during the last decade. The main emphasis of our research is to understand these interactions in detail, in particular non-conventional hydrogen bond acceptors. In this work, weakly bound complexes are studied using Pulsed Nozzle Fourier Transform Microwave Spectrometer, which has been fabricated in our laboratory and various theoretical methods. FTMW spectroscopy in the supersonic beam provides accurate structural information about the near-equilibrium geometry of small dimers and trimers in isolation. The home-built Pulsed Nozzle Microwave spectrometer, having a spectral range of 2-26.5 GHz has been used to record the microwave spectrum of these complexes. The spectrometer consists of a Fabry-Perot cavity, electronic circuit and pumps. Fabry-Perot cavity is the interaction zone of the molecules and radiation. The electronic circuit is used for the polarization and detection of the signal. Mechanical and diffusion pumps are used to maintain the vacuum inside the cavity. The gas molecules of interest are then mixed with a carrier gas and pulsed supersonically inside the cavity through a nozzle of 0.8 mm diameter. The emission from the complexes formed during the expansion is detected by super-heterodyne detection technique and then Fourier transformed. The first chapter of the thesis gives a brief introduction to intermolecular interactions, hydrogen bonding, halogen bonding, lithium bonding and molecular 2 of clusters. The chapter also includes a brief introduction to rotational spectroscopy. The second chapter of the thesis discusses the experimental and theoretical methods. It includes a detailed discussion of the mechanical and electrical parts of the spectrometer and the software used, which is developed in Labview 7.1. The various theoretical methods (ab initio and DFT) and the basis sets are discussed along with Atoms In Molecules Theory and the criteria used to characterize hydrogen bond. In the third chapter, to understand the ability of saturated hydrocarbons to act as hydrogen bond donor and acceptor, interaction of CH4 with H2S is studied using rotational spectroscopy as well as theoretical methods such as ab initio and Atoms In Molecules theory. Three progressions were obtained for the CH4•••H2S complex using microwave spectroscopy. The progressions were independently fitted to a linear top Hamiltonian. Absence of J10 transition in Progression II confirms the presence of higher internal angular momentum state, m=1. This also confirms the internal rotation of the monomers in the complex. Progressions II and III have negative centrifugal distortion constants. Hence both the states are from some excited internal rotation/torsional motion with strong vibrational-rotational coupling. The moment of inertia obtained from the experimental rotational constant confirms the structure in which sulphur of H2S is close to CH4. This also supports the structure in which CH4 is the hydrogen bond donor, if such an interaction is present. AIM analysis and the potential energy barrier for internal rotation show orientational preference and hence hydrogen bonding. The ab initio results show that CH4•••HSH interaction is more favorable than CH3H•••SH2. Ab initio and AIM studies also gave a structure where there is direct interaction between C and S. This is interesting since the electronegativities of C and S are comparable. Experimentally obtained negative distortion constants for the other two states, confirm excited state rotational-vibrational coupling. The experimental data give a floppy structure having internal rotation. In the fourth chapter the complex chosen for investigation is benzene-ethylene. Experiments in condensed phase and theoretical works show evidence of - stacking in benzene dimer, but there is no gas phase spectroscopic evidence available for the same. The lack of permanent dipole moment in the -stacked geometry of benzene dimer is the hindrance in the experimental observation of the same using microwave spectroscopy. Substitution of one of the benzene with ethylene in the -stacked structure will result in a complex having permanent dipole moment. C6H6 C2H4 complex can have, in addition to -stacking, C-H/interaction. There could be a competition between C6H6 and C2H4, either of which can act as H-bond donor. Experiments show the evidence of C-H/interaction, where C2H4 is the hydrogen bond donor. To ascertain hydrogen bond interaction AIM analysis has been carried out. The results show C-H/interaction, where one of the C2H4 hydrogen interacts with the benzene. Even though the aim was to get the -stacked geometry, it could not be obtained. However theory and AIM supports the formation of -stacked complex. In the fifth chapter using theoretical methods the ability of radicals as acceptor of hydrogen, lithium and chlorine bonds are examined with CF3 radical as the model system. As hydrogen bonds are highly sensitive to the environment, the effect of substitution of hydrogen by fluorine is also analyzed. It is found that, even though CH3 and CF3 radicals are topologically different, they interact in a similar fashion. AIM analysis of CF3HY satisfies all the eight criteria proposed by Koch and Popelier for hydrogen bonding. Here the hydrogen bond formed is charge transfer assisted. The interaction energies of the complexes are inversely proportional to the dipole moment of hydrogen bond donors and are proportional to the charge transfer occurring in the complex. Interaction energies from ab initio calculations confirm complexation of CF3 radical with LiY(Y=F, Cl, Br) and ClF. AIM analysis of CF3LiY and CF3ClF complexes show a bond critical point between Li/Cl and the C of CF3 and the condition of mutual penetration is also met. In CF3LiY complexes the interaction energies and charge transferred are directly proportional to the dipole moment of the Li bond donor. In the sixth chapter in order to extend the concept of non-conventional hydrogen bond acceptors to transition metals, complexes of Fe (Fe(CO)5) with HX (X=F,Cl,Br) have been studied theoretically. DFT calculations show that the structure in which the hydrogen of HX interacting with Fe through the sixth co-ordination site is a stable geometry. AIM analysis shows the presence of a bond critical point between the iron and the hydrogen of HX and hence bond formation. Q obtained from NBO analysis shows that there is charge transfer from the organometallic system to the hydrogen bond donor. However the interaction energies of the complexes are proportional to the dipole moment of hydrogen bond donors and are inversely proportional to the charge transfer for these complexes. H-bonding leads to the stabilization of square pyramidal geometry. ‘Hydrogen bond radius’ of iron has also been defined. Studies on the interaction of Fe(CO)5 with ClF and ClH showed that Fe can also act as a chlorine bond acceptor. Seventh chapter provides the overall conclusion and also discusses future direction.

The nature of interactions within a molecule, i.e. chemical bonding, is well understood today. However, our understanding about intermolecular interactions, which has great relevance in nature, is still evolving. Historically there are two types of intermolecular interactions, van der Waals interaction and hydrogen bonding. However, there has been an upsurge of interest in the halogen bonding and lithium bonding during the last decade. The main emphasis of our research is to understand these interactions in detail, in particular non-conventional hydrogen bond acceptors. In this work, weakly bound complexes are studied using Pulsed Nozzle Fourier Transform Microwave Spectrometer, which has been fabricated in our laboratory and various theoretical methods. FTMW spectroscopy in the supersonic beam provides accurate structural information about the near-equilibrium geometry of small dimers and trimers in isolation. The home-built Pulsed Nozzle Microwave spectrometer, having a spectral range of 2-26.5 GHz has been used to record the microwave spectrum of these complexes. The spectrometer consists of a Fabry-Perot cavity, electronic circuit and pumps. Fabry-Perot cavity is the interaction zone of the molecules and radiation. The electronic circuit is used for the polarization and detection of the signal. Mechanical and diffusion pumps are used to maintain the vacuum inside the cavity. The gas molecules of interest are then mixed with a carrier gas and pulsed supersonically inside the cavity through a nozzle of 0.8 mm diameter. The emission from the complexes formed during the expansion is detected by super-heterodyne detection technique and then Fourier transformed. The first chapter of the thesis gives a brief introduction to intermolecular interactions, hydrogen bonding, halogen bonding, lithium bonding and molecular 2 of clusters. The chapter also includes a brief introduction to rotational spectroscopy. The second chapter of the thesis discusses the experimental and theoretical methods. It includes a detailed discussion of the mechanical and electrical parts of the spectrometer and the software used, which is developed in Labview 7.1. The various theoretical methods (ab initio and DFT) and the basis sets are discussed along with Atoms In Molecules Theory and the criteria used to characterize hydrogen bond. In the third chapter, to understand the ability of saturated hydrocarbons to act as hydrogen bond donor and acceptor, interaction of CH4 with H2S is studied using rotational spectroscopy as well as theoretical methods such as ab initio and Atoms In Molecules theory. Three progressions were obtained for the CH4•••H2S complex using microwave spectroscopy. The progressions were independently fitted to a linear top Hamiltonian. Absence of J10 transition in Progression II confirms the presence of higher internal angular momentum state, m=1. This also confirms the internal rotation of the monomers in the complex. Progressions II and III have negative centrifugal distortion constants. Hence both the states are from some excited internal rotation/torsional motion with strong vibrational-rotational coupling. The moment of inertia obtained from the experimental rotational constant confirms the structure in which sulphur of H2S is close to CH4. This also supports the structure in which CH4 is the hydrogen bond donor, if such an interaction is present. AIM analysis and the potential energy barrier for internal rotation show orientational preference and hence hydrogen bonding. The ab initio results show that CH4•••HSH interaction is more favorable than CH3H•••SH2. Ab initio and AIM studies also gave a structure where there is direct interaction between C and S. This is interesting since the electronegativities of C and S are comparable. Experimentally obtained negative distortion constants for the other two states, confirm excited state rotational-vibrational coupling. The experimental data give a floppy structure having internal rotation. In the fourth chapter the complex chosen for investigation is benzene-ethylene. Experiments in condensed phase and theoretical works show evidence of - stacking in benzene dimer, but there is no gas phase spectroscopic evidence available for the same. The lack of permanent dipole moment in the -stacked geometry of benzene dimer is the hindrance in the experimental observation of the same using microwave spectroscopy. Substitution of one of the benzene with ethylene in the -stacked structure will result in a complex having permanent dipole moment. C6H6 C2H4 complex can have, in addition to -stacking, C-H/interaction. There could be a competition between C6H6 and C2H4, either of which can act as H-bond donor. Experiments show the evidence of C-H/interaction, where C2H4 is the hydrogen bond donor. To ascertain hydrogen bond interaction AIM analysis has been carried out. The results show C-H/interaction, where one of the C2H4 hydrogen interacts with the benzene. Even though the aim was to get the -stacked geometry, it could not be obtained. However theory and AIM supports the formation of -stacked complex. In the fifth chapter using theoretical methods the ability of radicals as acceptor of hydrogen, lithium and chlorine bonds are examined with CF3 radical as the model system. As hydrogen bonds are highly sensitive to the environment, the effect of substitution of hydrogen by fluorine is also analyzed. It is found that, even though CH3 and CF3 radicals are topologically different, they interact in a similar fashion. AIM analysis of CF3HY satisfies all the eight criteria proposed by Koch and Popelier for hydrogen bonding. Here the hydrogen bond formed is charge transfer assisted. The interaction energies of the complexes are inversely proportional to the dipole moment of hydrogen bond donors and are proportional to the charge transfer occurring in the complex. Interaction energies from ab initio calculations confirm complexation of CF3 radical with LiY(Y=F, Cl, Br) and ClF. AIM analysis of CF3LiY and CF3ClF complexes show a bond critical point between Li/Cl and the C of CF3 and the condition of mutual penetration is also met. In CF3LiY complexes the interaction energies and charge transferred are directly proportional to the dipole moment of the Li bond donor. In the sixth chapter in order to extend the concept of non-conventional hydrogen bond acceptors to transition metals, complexes of Fe (Fe(CO)5) with HX (X=F,Cl,Br) have been studied theoretically. DFT calculations show that the structure in which the hydrogen of HX interacting with Fe through the sixth co-ordination site is a stable geometry. AIM analysis shows the presence of a bond critical point between the iron and the hydrogen of HX and hence bond formation. Q obtained from NBO analysis shows that there is charge transfer from the organometallic system to the hydrogen bond donor. However the interaction energies of the complexes are proportional to the dipole moment of hydrogen bond donors and are inversely proportional to the charge transfer for these complexes. H-bonding leads to the stabilization of square pyramidal geometry. ‘Hydrogen bond radius’ of iron has also been defined. Studies on the interaction of Fe(CO)5 with ClF and ClH showed that Fe can also act as a chlorine bond acceptor. Seventh chapter provides the overall conclusion and also discusses future direction.

Weak intermolecular interactions have very strong impact on the structures and properties of life giving molecules like H2O, DNA, RNA etc. These interactions are responsible for many biological phenomena. The directional preference of some of these interactions is used for designing different synthetic approaches in the supramolecular chemistry. The work reported in this Thesis comprises of investigations of weak intermolecular interactions in gas phase using home-built Pulsed Nozzle Fourier Transform Microwave (PN-FTMW) spectrometer as an experimental tool and ab-initio and Atoms in Molecules (AIM) theory as theoretical tools. The spectrometer which is coupled with a pulsed nozzle is used to record pure rotational spectra of the molecular clusters in a jet cooled molecular beam. In the molecular beam molecules/complexes are free from interactions with other molecules/complexes and thus, spectroscopy in the molecular beams provides information about the 'isolated' molecule/complex under investigation. The rotational spectra of the molecules/complexes in the molecular beam provide their geometry in the ground vibrational states. These experimental geometries can be used to test the performance and accuracy of theoretical models like ab-initio theory, when applied to the weakly bound complexes. Further the AIM theory can be used to gain insights into the nature and strength of the intermolecular interactions present in the system under investigation. Chapter I of this Thesis gives a brief introduction of intermolecular interactions. Other than hydrogen bonding, which is considered as the most important intermolecular interaction, many other intermolecular interactions involving different atoms have been observed in past few decades. The chapter summarizes all these interactions. The chapter also gives a brief introduction to the experimental and theoretical methods used to probe these interactions. In Chapter II, the experimental and theoretical methods used in this work are summarized. Details of our home-built PN-FTMW spectrometer are given in this chapter. The chapter also discusses briefly the theoretical methods like ab-initio, AIM and Natural bond orbital (NBO) analysis. We have made few changes in the mode of control of one of our delay generators which have also been described. Chapter III and Chapter V of this Thesis are dedicated to the propargyl alcohol complexes. Propargyl alcohol (PA) is a molecule of astrophysical interest. It is also important in combustion chemistry since propargyl radical is considered as the precursor in soot formation. Moreover, PA is a multifunctional molecule, having a hydroxyl (-OH) and an acetylenic (-C≡C-H) group. Both of the groups can individually act as hydrogen bond acceptor as well as donor and thus PA provides an exciting possibility of studying many different types of weak interactions. Due to internal motion of -OH group, PA monomer can exist in gauche as well as trans form. However, rotational spectra of PA-monomer show the presence of only gauche conformer. In Chapter III, rotational spectra of Ar•••PA complex are discussed. The pure rotational spectra of the parent Ar•••PA complex and its two deuterated isotopologues, Ar•••PA-D (OD species) and Ar•••PA-D (CD species), could be observed and fitted within experimental uncertainty. The structural fitting confirmed a structure in which PA is present as gauche conformer and argon interacts with both the O-H group and the acetylenic group leading to Ar•••H-O and Ar•••π interactions respectively. Presence of these interactions was further confirmed by AIM theoretical analysis. In all the three isotopologues c-type rotational transitions showed significant splitting. Splitting patterns in the three isotopologues suggest that it originates mainly due to the large amplitude motion of the hydroxyl group and the motion is weakly coupled with the carbon chain bending motion. No evidence for the complex with trans conformer of PA was found. Although, we could not observe Ar•••trans-PA complex experimentally, we decided to perform ab-initio and AIM theoretical calculations on this complex as well. AIM calculations suggested the presence of Ar•••H-O and a unique Ar•••C interaction in this complex which was later found to be present in the Ar•••methanol complex as well. This prompted us to explore different possible interactions in methanol, other than the well known O-H•••O hydrogen bonding interactions, and eventually led us to an interesting interaction which we termed as carbon bond. Chapter IV discusses carbon bonding interaction in different complexes. Electrostatic potential (ESP) calculations show that tetrahedral face of methane is electron-rich and thus can act as hydrogen/halogen bond acceptor. This has already been observed in many complexes, e.g. CH4•••H2O/HF/HCl/ClF etc., both experimentally and theoretically. However, substitution of one of the hydrogens of methane with -OH leads to complete reversal of the properties of the CH3 tetrahedral face and this face in methanol is electron-deficient. We found that CH3 face in methanol interacts with electron rich sites of HnY molecules and leads to the formation of complexes stabilized by Y•••C-X interactions. This interaction was also found to be present in the complexes of many different CH3X (X=OH/F/Cl/Br/NO2/NF2 etc.) molecules. AIM, NBO and C-X frequency shift analyses suggest that this interaction could be termed as "carbon bond". The carbon bonding interactions could be important in understanding hydrophobic interactions and thus could play an important role in biological phenomena like protein folding. The carbon bonding interaction could also play a significant role in the stabilization of the transition state in SN2 reactions. In Chapter V of this Thesis rotational spectra of propargyl alcohol dimer are discussed. Rotational spectra of the parent dimer and its three deuterated (O-D) isotopologues (two mono-substituted and one bi-substituted) could be recorded and fitted within experimental uncertainty. The fitted rotational constants are close to one of the ab-initio predicted structure. In the dimer also propargyl alcohol exists in the gauche form. Atoms in molecules analysis suggests that the experimentally observed dimer is bound by O-H•••O, O-H•••π and C-H•••π interactions. Chapter VI of the thesis explores the 'electrophore concept'. To observe the rotational spectra of any species and determine its rotational constant by microwave spectroscopy, the species should have a permanent dipole moment. Can we obtain rotational constants of a species having no dipole moment via microwave spectroscopy? Electrophore concept can be used for this purpose. An electrophore is an atom or molecule which could combine with another molecule having no dipole moment thereby forming a complex with a dipole moment, e.g. Argon atom is an electrophore in Ar•••C6H6 complex. The microwave spectra of Ar•••13CC5H6 and Ar•••C6H5D complexes were recorded and fitted. The A rotational constant of these complexes was found to be equal to the C rotational constant of 13CC5H6 and C6H5D molecules respectively and thus we could determine the C rotational constant of microwave 'inactive' 13CC5H6. This concept could be used to obtain the rotational spectra of parallel displaced benzene-dimer if it exists. We recently showed that the square pyramidal Fe(CO)5 can act as hydrogen bond acceptor. Appendix I summarizes the extension of this work and discusses interactions of trigonal bipyramidal Fe(CO)5 with HF, HCl, HBr and ClF. Our initial attempts on generating a chirped pulse to be used in a new broadband spectrometer are summarized in Appendix II. Preliminary investigations on the propargyl•••water complex are summarized in Appendix III.

Weak intermolecular interactions have very strong impact on the structures and properties of life giving molecules like H2O, DNA, RNA etc. These interactions are responsible for many biological phenomena. The directional preference of some of these interactions is used for designing different synthetic approaches in the supramolecular chemistry. The work reported in this Thesis comprises of investigations of weak intermolecular interactions in gas phase using home-built Pulsed Nozzle Fourier Transform Microwave (PN-FTMW) spectrometer as an experimental tool and ab-initio and Atoms in Molecules (AIM) theory as theoretical tools. The spectrometer which is coupled with a pulsed nozzle is used to record pure rotational spectra of the molecular clusters in a jet cooled molecular beam. In the molecular beam molecules/complexes are free from interactions with other molecules/complexes and thus, spectroscopy in the molecular beams provides information about the 'isolated' molecule/complex under investigation. The rotational spectra of the molecules/complexes in the molecular beam provide their geometry in the ground vibrational states. These experimental geometries can be used to test the performance and accuracy of theoretical models like ab-initio theory, when applied to the weakly bound complexes. Further the AIM theory can be used to gain insights into the nature and strength of the intermolecular interactions present in the system under investigation. Chapter I of this Thesis gives a brief introduction of intermolecular interactions. Other than hydrogen bonding, which is considered as the most important intermolecular interaction, many other intermolecular interactions involving different atoms have been observed in past few decades. The chapter summarizes all these interactions. The chapter also gives a brief introduction to the experimental and theoretical methods used to probe these interactions. In Chapter II, the experimental and theoretical methods used in this work are summarized. Details of our home-built PN-FTMW spectrometer are given in this chapter. The chapter also discusses briefly the theoretical methods like ab-initio, AIM and Natural bond orbital (NBO) analysis. We have made few changes in the mode of control of one of our delay generators which have also been described. Chapter III and Chapter V of this Thesis are dedicated to the propargyl alcohol complexes. Propargyl alcohol (PA) is a molecule of astrophysical interest. It is also important in combustion chemistry since propargyl radical is considered as the precursor in soot formation. Moreover, PA is a multifunctional molecule, having a hydroxyl (-OH) and an acetylenic (-C≡C-H) group. Both of the groups can individually act as hydrogen bond acceptor as well as donor and thus PA provides an exciting possibility of studying many different types of weak interactions. Due to internal motion of -OH group, PA monomer can exist in gauche as well as trans form. However, rotational spectra of PA-monomer show the presence of only gauche conformer. In Chapter III, rotational spectra of Ar•••PA complex are discussed. The pure rotational spectra of the parent Ar•••PA complex and its two deuterated isotopologues, Ar•••PA-D (OD species) and Ar•••PA-D (CD species), could be observed and fitted within experimental uncertainty. The structural fitting confirmed a structure in which PA is present as gauche conformer and argon interacts with both the O-H group and the acetylenic group leading to Ar•••H-O and Ar•••π interactions respectively. Presence of these interactions was further confirmed by AIM theoretical analysis. In all the three isotopologues c-type rotational transitions showed significant splitting. Splitting patterns in the three isotopologues suggest that it originates mainly due to the large amplitude motion of the hydroxyl group and the motion is weakly coupled with the carbon chain bending motion. No evidence for the complex with trans conformer of PA was found. Although, we could not observe Ar•••trans-PA complex experimentally, we decided to perform ab-initio and AIM theoretical calculations on this complex as well. AIM calculations suggested the presence of Ar•••H-O and a unique Ar•••C interaction in this complex which was later found to be present in the Ar•••methanol complex as well. This prompted us to explore different possible interactions in methanol, other than the well known O-H•••O hydrogen bonding interactions, and eventually led us to an interesting interaction which we termed as carbon bond. Chapter IV discusses carbon bonding interaction in different complexes. Electrostatic potential (ESP) calculations show that tetrahedral face of methane is electron-rich and thus can act as hydrogen/halogen bond acceptor. This has already been observed in many complexes, e.g. CH4•••H2O/HF/HCl/ClF etc., both experimentally and theoretically. However, substitution of one of the hydrogens of methane with -OH leads to complete reversal of the properties of the CH3 tetrahedral face and this face in methanol is electron-deficient. We found that CH3 face in methanol interacts with electron rich sites of HnY molecules and leads to the formation of complexes stabilized by Y•••C-X interactions. This interaction was also found to be present in the complexes of many different CH3X (X=OH/F/Cl/Br/NO2/NF2 etc.) molecules. AIM, NBO and C-X frequency shift analyses suggest that this interaction could be termed as "carbon bond". The carbon bonding interactions could be important in understanding hydrophobic interactions and thus could play an important role in biological phenomena like protein folding. The carbon bonding interaction could also play a significant role in the stabilization of the transition state in SN2 reactions. In Chapter V of this Thesis rotational spectra of propargyl alcohol dimer are discussed. Rotational spectra of the parent dimer and its three deuterated (O-D) isotopologues (two mono-substituted and one bi-substituted) could be recorded and fitted within experimental uncertainty. The fitted rotational constants are close to one of the ab-initio predicted structure. In the dimer also propargyl alcohol exists in the gauche form. Atoms in molecules analysis suggests that the experimentally observed dimer is bound by O-H•••O, O-H•••π and C-H•••π interactions. Chapter VI of the thesis explores the 'electrophore concept'. To observe the rotational spectra of any species and determine its rotational constant by microwave spectroscopy, the species should have a permanent dipole moment. Can we obtain rotational constants of a species having no dipole moment via microwave spectroscopy? Electrophore concept can be used for this purpose. An electrophore is an atom or molecule which could combine with another molecule having no dipole moment thereby forming a complex with a dipole moment, e.g. Argon atom is an electrophore in Ar•••C6H6 complex. The microwave spectra of Ar•••13CC5H6 and Ar•••C6H5D complexes were recorded and fitted. The A rotational constant of these complexes was found to be equal to the C rotational constant of 13CC5H6 and C6H5D molecules respectively and thus we could determine the C rotational constant of microwave 'inactive' 13CC5H6. This concept could be used to obtain the rotational spectra of parallel displaced benzene-dimer if it exists. We recently showed that the square pyramidal Fe(CO)5 can act as hydrogen bond acceptor. Appendix I summarizes the extension of this work and discusses interactions of trigonal bipyramidal Fe(CO)5 with HF, HCl, HBr and ClF. Our initial attempts on generating a chirped pulse to be used in a new broadband spectrometer are summarized in Appendix II. Preliminary investigations on the propargyl•••water complex are summarized in Appendix III.

The importance of weak interactions between molecules to life and all parts of science and engineering is unquestionable and there have been an enormous interest in such interactions. Among all the weak interactions, hydrogen bonding is the most popular and it has enjoyed the most attention of the scientific community. Halogen bonding is gaining more popularity in the recent time, as its importance to biological molecules and crystal engineering has been recognized. In this work, a Pulsed Nozzle Fourier Transform Microwave spectrometer has been used to study the rotational spectra of molecules and hydrogen bonded complexes. Structural information is obtained from the rotational spectra. Ab initio electronic structure, Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) theoretical methods have been used to characterize the weak intermolecular interactions, including hydrogen bonding, halogen bonding and lithium bonding. In Chapter I, introduction to weak interaction is discussed. A brief introduction of different experimental and theoretical methods is presented. Chapter II discusses in detail about the different methods used to investigate weak interaction, both experimentally and theoretically, in this work. In our lab, we use Pulsed Nozzle Fourier Transform Microwave spectrometer to determine the complexes spectra and structures. We generate MW radiation with the help of electronic devices and use Balle-Flygare cavity where molecular interaction takes place. We inject the sample inside the cavity in form of supersonic molecular beam through a pulsed nozzle, parallel to MW radiation. The detailed instrumental discussion about MW spectrometer has been done in this Chapter. We extensively use theoretical methods to probe weak bonding and characterize them. Ab initio and DFT calculations are used to optimize the structure of the complexes and predict their rotational spectra. Atoms in Molecules theory and Natural Bond Orbital theory are then used with the ab initio wave functions to understand the weak interactions in depth. Discussion about these methods and software used for the analysis will also be discussed. In Chapter III, rotational spectrum of Hexafluoroisopropanol (HFIP) monomer is presented. HFIP is an interesting molecule as it offers many possibilities as hydrogen bond donor and acceptor. It has the OH group which can both accept/donate a hydrogen bond and in addition it has a very acidic CH group. It is the only solvent that can dissolve polyethylene terephthalate, a normally difficult-to-dissolve polymer, and clearly it has unique interactions with this difficult to solve polymer. We have recorded and fitted rotational spectra of five different isotopologues of HFIP which helped us in determining its accurate structure. Though, it can exist in synclinical and antiperiplanar conformers, only the later has been detected in our molecular beam spectrometer. This happens to be the global minimum structure of HFIP. Combination of experimental observations and ab initio calculations provided many evidences which confirmed the presence of antiperiplanar conformer, experimentally. Since, the rotational constants for both conformers were very close, it was always challenging to pick up one conformer as experimentally observed structure. A prototype molecule, hexafluoroisobutene (HFIB) shows doubling of rotational transitions due to tunnelling/counter rotation of the two CF3 groups through a small barrier. Interestingly, such motion has no barrier in HFIP and hence no splitting in transitions was observed. Potential energy surface calculated for counter-rotation of the two CF3 groups is consistent with this observation. This barrier is different from eclipsed-staggered exchange barrier, observed by 60 counter rotation of both terminal CF3 groups, for which the barrier height is very large and tunnelling cannot occur. The origin/lack of the small barrier in HFIB/HFIP has been explored using Natural Bond Orbital (NBO) method which helped in understanding intramolecular bonding in these molecules. Along with HFIB, other prototype molecules were also considered for the analysis e.g. hexafluoroacetone, hexafluoroacetone imine, hexafluoroisobutane, hexafluoroisopropylamine. In the last section of this Chapter, we have discussed the generalized behaviour of molecules which have CF3-C-CF3 groups. In Chapter IV, rotational spectrum of HFIP•••H2O complex is presented. Aqueous solution of HFIP stabilizes α-helical structure of protein, a unique property of this solvent. The main objective of this Chapter is understanding the interaction between HFIP and H2O. Microwave spectrum of HFIP•••H2O was predicted and recorded. Three isotopologues were investigated. Though, this complex could in principle have several structural conformers, detailed ab initio calculations predicted two conformers and only one was observed. Though, the rotational constants for both structures were somewhat similar, lack of a dipole transitions, larger intensity of b-dipole transitions over c-dipole transitions and isotopic substitution analysis positively confirm the structure in which HFIP acts as the hydrogen bond donor. The linear O-H•••O hydrogen bond in HFIP-H2O complex is significantly stronger than that in water dimer with the H•••O distance of 1.8 Å. The other structure for this complex, not found in experiment is cyclic with both C-H•••O and O-H•••O hydrogen bonds, both of which are bent with H•••O distances in the range 2.2-2.3 Å. Both AIM and NBO calculations have been used to characterize the hydrogen bond in this complex. In Chapter V, a comprehensive study on hydrogen bonding, chlorine bonding and lithium bonding have been done. A typical hydrogen bonded complex can be represented as A•••H-D, where A is the acceptor unit and H-D is the hydrogen bond donor unit. Many examples are known in literature, both experimentally and theoretically, in which the A-H-D bond angles are not linear. Deviation from linearity also results in the increase in A•••H bond lengths, as noted above for the two structures of HFIP•••H2O complex. Though this has been known for long, the distance between A and D being less than the sum of their van der Waals ‘radii’ is still used as a criterion for hydrogen bonding by many. Our group has recently shown the inappropriateness of van der Waals ‘radii’ and defined hydrogen bond ‘radii’ for various donors, DH and A. A strong correlation of DH hydrogen bond ‘radii’ with the dipole moment was noted. In this Chapter, we explored in detail the angular dependence of hydrogen bond ‘radii’. Electron density topology around DH (D = F, Cl and OH) has been analyzed in detail and shown to be elliptical. For these molecules, the two constants for H atom treated as an ellipse have been determined. It is hoped that these two constants will be used widely in analyzing and interpreting H•••A distances, as a function of D-H•••A angles, rather than one ‘radius’ for H and acceptor atoms. In Chapter VI, Detailed analysis and comparisons among hydrogen bond, chlorine bond and lithium bond, have been done. Hydrogen can be placed in group 1 as well as group 17 of the periodic table. Naturally, lithium bonding and halogen bonding have been proposed and investigated. There have been numerous investigations on the nature of hydrogen bonding and the physical forces contributing to it. In this Chapter, a total of one hundred complexes having H/Cl/Li bonding have been investigated using ab initio, AIM and NBO theoretical methods. Various criteria proposed in the literature have been examined. A new criterion has been proposed for the characterization of closed shell (ionic/electrostatic) and open shell (covalent) interactions. It has been well known that the D-H bond weakens on the D-H•••A hydrogen bond formation and H•••A bond acquires a fractional covalency. This Chapter shows that for D-Li•••A complexes, the ionicity in D-Li is reduced as the Li•••A bond is formed This comprehensive investigation of H/Cl/Li bonding has led us to propose a conservation of bond order, considering both ionic and covalent contributions to both D-X and X•••A bonds, where DX is the X-bond donor and A is the acceptor with X = H/Cl/Li. Hydrogen bond is well understood and its definition has been recently revised [Arunan et al. Pure Appl. Chem., Vol. 83, pp. 1619–1636, 2011]. It states “The X–H•••Y hydrogen bond angle tends toward 180° and should preferably be above 110°”. Using AIM theory and other methods, this fact is examined and presented in Appendix A. In second part of appendix A, a discussion about calling H3¯ complex as trihydrogen bond and its comparison with FHF¯ complex, is presented. In Appendix B, there is tentative prediction and discussion about the HFIP dimer. Condense phase studies show that HFIP have strong aggregation power to form dimer, trimer etc. During, HFIP monomer study, we have unassigned lines which are suspected to be from HFIP dimer. These are tabulated in the Appendix B as well.

The importance of weak interactions between molecules to life and all parts of science and engineering is unquestionable and there have been an enormous interest in such interactions. Among all the weak interactions, hydrogen bonding is the most popular and it has enjoyed the most attention of the scientific community. Halogen bonding is gaining more popularity in the recent time, as its importance to biological molecules and crystal engineering has been recognized. In this work, a Pulsed Nozzle Fourier Transform Microwave spectrometer has been used to study the rotational spectra of molecules and hydrogen bonded complexes. Structural information is obtained from the rotational spectra. Ab initio electronic structure, Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) theoretical methods have been used to characterize the weak intermolecular interactions, including hydrogen bonding, halogen bonding and lithium bonding. In Chapter I, introduction to weak interaction is discussed. A brief introduction of different experimental and theoretical methods is presented. Chapter II discusses in detail about the different methods used to investigate weak interaction, both experimentally and theoretically, in this work. In our lab, we use Pulsed Nozzle Fourier Transform Microwave spectrometer to determine the complexes spectra and structures. We generate MW radiation with the help of electronic devices and use Balle-Flygare cavity where molecular interaction takes place. We inject the sample inside the cavity in form of supersonic molecular beam through a pulsed nozzle, parallel to MW radiation. The detailed instrumental discussion about MW spectrometer has been done in this Chapter. We extensively use theoretical methods to probe weak bonding and characterize them. Ab initio and DFT calculations are used to optimize the structure of the complexes and predict their rotational spectra. Atoms in Molecules theory and Natural Bond Orbital theory are then used with the ab initio wave functions to understand the weak interactions in depth. Discussion about these methods and software used for the analysis will also be discussed. In Chapter III, rotational spectrum of Hexafluoroisopropanol (HFIP) monomer is presented. HFIP is an interesting molecule as it offers many possibilities as hydrogen bond donor and acceptor. It has the OH group which can both accept/donate a hydrogen bond and in addition it has a very acidic CH group. It is the only solvent that can dissolve polyethylene terephthalate, a normally difficult-to-dissolve polymer, and clearly it has unique interactions with this difficult to solve polymer. We have recorded and fitted rotational spectra of five different isotopologues of HFIP which helped us in determining its accurate structure. Though, it can exist in synclinical and antiperiplanar conformers, only the later has been detected in our molecular beam spectrometer. This happens to be the global minimum structure of HFIP. Combination of experimental observations and ab initio calculations provided many evidences which confirmed the presence of antiperiplanar conformer, experimentally. Since, the rotational constants for both conformers were very close, it was always challenging to pick up one conformer as experimentally observed structure. A prototype molecule, hexafluoroisobutene (HFIB) shows doubling of rotational transitions due to tunnelling/counter rotation of the two CF3 groups through a small barrier. Interestingly, such motion has no barrier in HFIP and hence no splitting in transitions was observed. Potential energy surface calculated for counter-rotation of the two CF3 groups is consistent with this observation. This barrier is different from eclipsed-staggered exchange barrier, observed by 60 counter rotation of both terminal CF3 groups, for which the barrier height is very large and tunnelling cannot occur. The origin/lack of the small barrier in HFIB/HFIP has been explored using Natural Bond Orbital (NBO) method which helped in understanding intramolecular bonding in these molecules. Along with HFIB, other prototype molecules were also considered for the analysis e.g. hexafluoroacetone, hexafluoroacetone imine, hexafluoroisobutane, hexafluoroisopropylamine. In the last section of this Chapter, we have discussed the generalized behaviour of molecules which have CF3-C-CF3 groups. In Chapter IV, rotational spectrum of HFIP•••H2O complex is presented. Aqueous solution of HFIP stabilizes α-helical structure of protein, a unique property of this solvent. The main objective of this Chapter is understanding the interaction between HFIP and H2O. Microwave spectrum of HFIP•••H2O was predicted and recorded. Three isotopologues were investigated. Though, this complex could in principle have several structural conformers, detailed ab initio calculations predicted two conformers and only one was observed. Though, the rotational constants for both structures were somewhat similar, lack of a dipole transitions, larger intensity of b-dipole transitions over c-dipole transitions and isotopic substitution analysis positively confirm the structure in which HFIP acts as the hydrogen bond donor. The linear O-H•••O hydrogen bond in HFIP-H2O complex is significantly stronger than that in water dimer with the H•••O distance of 1.8 Å. The other structure for this complex, not found in experiment is cyclic with both C-H•••O and O-H•••O hydrogen bonds, both of which are bent with H•••O distances in the range 2.2-2.3 Å. Both AIM and NBO calculations have been used to characterize the hydrogen bond in this complex. In Chapter V, a comprehensive study on hydrogen bonding, chlorine bonding and lithium bonding have been done. A typical hydrogen bonded complex can be represented as A•••H-D, where A is the acceptor unit and H-D is the hydrogen bond donor unit. Many examples are known in literature, both experimentally and theoretically, in which the A-H-D bond angles are not linear. Deviation from linearity also results in the increase in A•••H bond lengths, as noted above for the two structures of HFIP•••H2O complex. Though this has been known for long, the distance between A and D being less than the sum of their van der Waals ‘radii’ is still used as a criterion for hydrogen bonding by many. Our group has recently shown the inappropriateness of van der Waals ‘radii’ and defined hydrogen bond ‘radii’ for various donors, DH and A. A strong correlation of DH hydrogen bond ‘radii’ with the dipole moment was noted. In this Chapter, we explored in detail the angular dependence of hydrogen bond ‘radii’. Electron density topology around DH (D = F, Cl and OH) has been analyzed in detail and shown to be elliptical. For these molecules, the two constants for H atom treated as an ellipse have been determined. It is hoped that these two constants will be used widely in analyzing and interpreting H•••A distances, as a function of D-H•••A angles, rather than one ‘radius’ for H and acceptor atoms. In Chapter VI, Detailed analysis and comparisons among hydrogen bond, chlorine bond and lithium bond, have been done. Hydrogen can be placed in group 1 as well as group 17 of the periodic table. Naturally, lithium bonding and halogen bonding have been proposed and investigated. There have been numerous investigations on the nature of hydrogen bonding and the physical forces contributing to it. In this Chapter, a total of one hundred complexes having H/Cl/Li bonding have been investigated using ab initio, AIM and NBO theoretical methods. Various criteria proposed in the literature have been examined. A new criterion has been proposed for the characterization of closed shell (ionic/electrostatic) and open shell (covalent) interactions. It has been well known that the D-H bond weakens on the D-H•••A hydrogen bond formation and H•••A bond acquires a fractional covalency. This Chapter shows that for D-Li•••A complexes, the ionicity in D-Li is reduced as the Li•••A bond is formed This comprehensive investigation of H/Cl/Li bonding has led us to propose a conservation of bond order, considering both ionic and covalent contributions to both D-X and X•••A bonds, where DX is the X-bond donor and A is the acceptor with X = H/Cl/Li. Hydrogen bond is well understood and its definition has been recently revised [Arunan et al. Pure Appl. Chem., Vol. 83, pp. 1619–1636, 2011]. It states “The X–H•••Y hydrogen bond angle tends toward 180° and should preferably be above 110°”. Using AIM theory and other methods, this fact is examined and presented in Appendix A. In second part of appendix A, a discussion about calling H3¯ complex as trihydrogen bond and its comparison with FHF¯ complex, is presented. In Appendix B, there is tentative prediction and discussion about the HFIP dimer. Condense phase studies show that HFIP have strong aggregation power to form dimer, trimer etc. During, HFIP monomer study, we have unassigned lines which are suspected to be from HFIP dimer. These are tabulated in the Appendix B as well.

There are currently three known crystal packing motifs of boronsubphthalocyanine derivatives. Each motif is associated with a particular class of BsubPc derivatives, and none are ideal for organic electronic applications according to the criteria we defined for evaluation: having a continuous pathway for charge-carrier conduction in the solid-state, resistance to hydrolysis, good electrochemical and optical properties, and possession of a robust crystal form. In this thesis, we present five methods for altering the crystal packing structure of phenoxy-BsubPc derivatives in order to meet the above four criteria. We find that neither addition of steric bulk to the axial derivative nor changing the symmetry of the compounds is sufficient for creating a new crystal packing motif. We do find that reducing the symmetry of the axial group does increase the solubility greatly, however. We identify a new motif for BsubPc crystals that occurs when the intermolecular interactions between the axial phenoxy segment and the BsubPc ligand are increased. We present two methods for achieving this new motif, one is through addition of a π-Br interaction and the other is through creation of a strong π-acid/ π-base stacking by making the axial phenoxy more π-electron rich. Unfortunately, the p-bromophenoxy-BsubPc forms this new motif as a kinetic product, isolation of which is unreliable. Attaching a naphthol fragment axially to the BsubPc creates a stable version of this new motif. We also synthesized a new class of BsubPc pseudohalides based on sulfonate derivatives. Of the derivatives in this new class, we found that mesylate-BsubPc forms into a crystal packing structure that possesses a one-dimensional pathway for charge carrier mobility, but is still resistant to hydrolysis under the conditions tested. Overall, we show four compounds that meet the criteria for further study as organic electronic materials: p-methoxyphenoxy-BsubPc, α-naphthoxy-BsubPc, β-naphthoxy-BsubPc, and mesylate-BsubPc.

Atoms construct the molecules and molecules construct the material substances (with the exceptions as well, e.g.., metals, where atoms directly construct the material substances). Intermolecular interactions play an important role in most of the branches of sciences, ranging from material sciences to biological sciences. Van der Waals interactions are weak intermolecular interactions while hydrogen bonding varies in strength from weak to strong (1 to 40 kcal/mol). The present work focuses on applying some theoretical methods (ab initio and Atoms in Molecules theory) on these interactions to differentiate them with physically meaningful parameters such as hydrogen bond radii and atoms in molecules theory parameters. 1)Defining and calculating H-bond radii have been done using atoms in molecules theory approach which can explain ruling out the presence or absence of an H-bond in an intermolecular interaction. 2) A blue-shift of 200 cm-1 for a weakly bound complex is unprecedented. Our studies on weakly bound complexes showed the blue-shift of 200 cm-1 for H3C•••CIF and shift has been found to be purely from the mixing of normal modes and not because of an interaction. 3)Methane, a symmetric top molecule can act both as H-bond acceptor and donor. The present work shows that methane is rather a better H-bond acceptor than a donor and all the calculated parameters are in favor of this description. 4) Microwave spectrometer is an ultimate tool (at least at present) for structural characterization of the weakly bound complexes accurately. The rotational spectrum of the weakly bound isotopomer weakly bound complexes accurately. The rotational spectrum of the weakly bound isotopomer 13CC5H6•••Ar, which is a symmetric top and gives only “B” rotational constant. Moreover, the A rotational constant of the complex is the same as the rotational constant for 13CC5H6, which has no dipole moment. C2H2 molecule is an astrophysically important molecule as it is present in asymptotic giant branch and T-type stars (Teff<3000K). Due to its various infrared active vibrational modes, C2H2 is one of the most important sources in cool stars. The production of C2H2 infrared spectroscopic data at high temperature is therefore essential to trace back physical characteristics of these objects and to model the radiative transfer in their envelope. The databases such as “HITRAN”, do not have enough data available for stimulating high temperature spectra. Keeping all these objectives in mind, high temperature emission spectrum of acetylene has been recorded around 3µm region of acetylene.

Atoms construct the molecules and molecules construct the material substances (with the exceptions as well, e.g.., metals, where atoms directly construct the material substances). Intermolecular interactions play an important role in most of the branches of sciences, ranging from material sciences to biological sciences. Van der Waals interactions are weak intermolecular interactions while hydrogen bonding varies in strength from weak to strong (1 to 40 kcal/mol). The present work focuses on applying some theoretical methods (ab initio and Atoms in Molecules theory) on these interactions to differentiate them with physically meaningful parameters such as hydrogen bond radii and atoms in molecules theory parameters. 1)Defining and calculating H-bond radii have been done using atoms in molecules theory approach which can explain ruling out the presence or absence of an H-bond in an intermolecular interaction. 2) A blue-shift of 200 cm-1 for a weakly bound complex is unprecedented. Our studies on weakly bound complexes showed the blue-shift of 200 cm-1 for H3C•••CIF and shift has been found to be purely from the mixing of normal modes and not because of an interaction. 3)Methane, a symmetric top molecule can act both as H-bond acceptor and donor. The present work shows that methane is rather a better H-bond acceptor than a donor and all the calculated parameters are in favor of this description. 4) Microwave spectrometer is an ultimate tool (at least at present) for structural characterization of the weakly bound complexes accurately. The rotational spectrum of the weakly bound isotopomer weakly bound complexes accurately. The rotational spectrum of the weakly bound isotopomer 13CC5H6•••Ar, which is a symmetric top and gives only “B” rotational constant. Moreover, the A rotational constant of the complex is the same as the rotational constant for 13CC5H6, which has no dipole moment. C2H2 molecule is an astrophysically important molecule as it is present in asymptotic giant branch and T-type stars (Teff<3000K). Due to its various infrared active vibrational modes, C2H2 is one of the most important sources in cool stars. The production of C2H2 infrared spectroscopic data at high temperature is therefore essential to trace back physical characteristics of these objects and to model the radiative transfer in their envelope. The databases such as “HITRAN”, do not have enough data available for stimulating high temperature spectra. Keeping all these objectives in mind, high temperature emission spectrum of acetylene has been recorded around 3µm region of acetylene.

Dość pobieżnie dotąd scharakteryzowane hybrydowe organiczno-nieorganiczne półprzewodniki są interesującą grupą materiałów o potencjalnym zastosowaniu w elektronice, fotokatalizie, fotowoltaice, optoelektronice i pokrewnych dziedzinach badań. Siła oddziaływań obecnych w strukturze krystalicznej znajduje odzwierciedlenie w strukturze elektronowej, co można zaobserwować analizując dystrybucję gęstości stanów elektronowych poszczególnych materiałów. Silne oddziaływania (kowalencyjne i jonowe) w półprzewodnikach nieorganicznych pozwalają na tworzenie się ciągłych pasm w strukturze elektronowej. Po drugiej stronie stoją półprzewodniki czysto organiczne których, właściwości zależą w głównej mierze od słabych oddziaływań międzycząsteczkowych. Dużo słabsze oddziaływania van der Waalsa, wiązania wodorowe i oddziaływania typu π prowadzą do powstania wąskich pasm energetycznych. W przypadku półprzewodników hybrydowych oba przypadki są możliwe, a przewaga słabych lub silnych oddziaływań w strukturze krystalicznej determinuje właściwości materiału. Półprzewodniki hybrydowe reprezentowane są w niniejszej pracy przez tetrajodkowe kompleksy cyny z ligandami organicznymi ([SnI4{(C6H5)3PO}2], [SnI4{(C6H5)2SO}2] oraz [SnI4(C5H5NO)2]). Obecny w nich atom jodu charakteryzuje się dużym promieniem oraz polaryzowalnością, co sprzyja tworzeniu się słabszych wiązań i oddziaływań międzycząsteczkowych. W przypadku tych kompleksów główną rolę odgrywają oddziaływania wodorowe z atomami jodu łączące fragmenty organiczne i nieorganiczne sąsiadujących cząsteczek. Im słabsze oddziaływania międzycząsteczkowe w strukturze krystalicznej tym dyskretniejsza struktura pasmowa i bardziej płaskie pasma w strukturze elektronowej. Jeśli oddziaływania te są odpowiednio słabe może dojść do powstania przypadku granicznego, w którym materiał może zostać sklasyfikowany jako izolator posiadający jednak jeszcze pewne cechy półprzewodnika. Przykładami tego typu materiałów są opisane w rozprawie organiczne sole trójjodkowe o dużych kationach organicznych. W ich strukturze krystalicznej brak silnych oddziaływań międzycząsteczkowych, a liniowo ułożone aniony I3 - są otoczone przez kationy zawierające liczne podstawniki aromatyczne. Mimo niewielkiej siły oddziaływania pomiędzy anionami trójjodkowymi orbitale atomów jodu budują zarówno krawędzie pasm walencyjnych jak i przewodnictwa. Inne słabe oddziaływania obecne w strukturze to oddziaływania wodorowe CH…I (w większości przypadków odległości H…I są zbyt duże aby nazywać je wiązaniami). Mimo niewielkiej mobilności nośników ładunku materiały te wydajnie generują fotoprądy w zakresie promieniowania ultrafioletowego, ponieważ zachodzi wówczas wzbudzenie elektronów do wyższych pasm przewodnictwa, na które składają się orbitale atomów węgla. Materiałem opierającym się głównie na silnych wiązaniach kowalencyjnych jest jodek ołowiu(II) - półprzewodnik szeroko opisany w literaturze. Głównym aspektem poruszanym w rozprawie jest tworzenie złącza Schottky'ego przez halogenki ołowiu(II) w kontakcie z powierzchnią niektórych metali. Stany powierzchniowe prowadzące do powstania bariery Schottky'ego mogą mieć swoje źródła w chemisorpcji cząsteczek półprzewodnika na powierzchni metalu, co może równocześnie powodować tzw. "pillow effect" przejawiający się redukcją pracy wyjścia metalu. Innym istotnym źródłem stanów powierzchniowych jest nakładanie się funkcji elektronowej metalu na stany obecne w przerwie energetycznej półprzewodnika wynikające ze skończonych rozmiarów kryształów. Niesymetryczne zapełnianie i opróżnianie stanów powierzchniowych obecnych na złączu PbI2|Cu zostało wykorzystane do zbudowania memrystora - elementu, który w wyniku przepływu ładunku zmienia swój opór. W pracy oprócz analizy mechanizmu działania tego urządzenia pokazano podobieństwo zbudowanego memrystora do synapsy oraz zastosowanie memrystora do prostej analizy sygnału. Opisany w pracy memrystor bazujący na złączu PbI2|Cu został wbudowany w układ rezerwuarowy - podtyp sieci neuronowej. Pozwoliło to na wykonanie prostych obliczeń, tj. na klasyfikację sygnałów ze względu na ich amplitudę. Innym zjawiskiem zaobserwowanym w tym układzie jest metaplastyczność. Efekt ten przejawia się jako zmiana podstawowych właściwości urządzenia ze względu na jego historię (tj. dłuższą ekspozycję na dodatni potencjał i związany z nim przepływ ładunku) i jest możliwy dzięki tworzeniu się dodatkowych oddziaływań pomiędzy metalem a półprzewodnikiem, które zmieniają opór takiego złącza. Źródłem tych nowych oddziaływań może być utlenianie atomów miedzi, które następnie mogą utworzyć wiązanie z obecnymi na powierzchni PbI2 atomami jodu. Działanie całego układu sprowadza się więc do oddziaływań powierzchni halogenku ołowiu(II) z powierzchnią metalu. W skład rozprawy wchodzi pięć publikacji poświęconych różnym aspektom fizykochemii półprzewodników oraz przetwarzaniu informacji. Trzy z tych publikacji koncentrują się na oddziaływaniach występujących w strukturach krystalicznych związków lub na granicy półprzewodnik/metal. Poddawane analizie związki różnią się znacząco składem i budową: 1) organiczne sole trójjodkowe oprócz anionu I3-zawierają duże kationy organiczne zawierające trzy lub sześć pierścieni aromatycznych; 2) jodkowe kompleksy cyny(IV) mają budowę molekularną a atom jodu jest związany jednym wiązaniem kowalencyjnym z atomem cyny; 3) jodek ołowiu(II) ma budowę warstwową, w której atom jodu ma rolę mostka łączącego sąsiednie atomy ołowiu. Odmienność przedstawionych struktur pozwala na prześledzenie różnorodnych oddziaływań - od silnych wiązań kowalencyjnych, jonowych przez słabsze π-π, wodorowe lub halogenowe, na oddziaływaniach van der Waalsa kończąc. Artykuły opisujące właściwości kompleksów SnI4 oraz soli trójjodkowych skupiają się na wpływie oddziaływania poszczególnych fragmentów struktury krystalicznej na strukturę elektronową, przewodnictwo, przerwę energetyczna, mobilność i koncentrację nośników ładunku, typ domieszkowania oraz potencjały krawędzi pasm. W publikacjach tych wykazane jest, że słabsze oddziaływania obecne w strukturze w dużych ilościach potrafią mieć dominujący wpływ na strukturę elektronową związku. Wyciągnięte wnioski poparte są między innymi pomiarami spektroktroskopowymi oraz obliczeniami teoretycznymi. W tych dwóch publikacjach zaprezentowano cztery nowe związki: [(C6H5CH2)3NO]2H+I3-, (C6H5CH2)3NH+I3- ∙ C6H5CH3, [SnI4{(C6H5)2SO}2] i [SnI4(C5H5NO)2] o nieopisanych wcześniej strukturach krystalicznych. Z kolei publikacja skupiająca się na urządzeniu zbudowanym na bazie PbI2 opisuje oddziaływania tego materiału z powierzchnią różnych metali (Au, Pt, Cu, Ag i Al) oraz wyjaśnia wpływ tego oddziaływania na barierę Schottky'ego, odpowiedzialną za działanie tego urządzenia. Rozprawę uzupełniają dwie prace przeglądowe, które dopełniają dwa nurty podjęte w pracy: pierwszy dotyczący analizy właściwości konkretnej grupy półprzewodników (chalkohalogenków) oraz drugi związany z budowaniem urządzeń na podstawie materiałów półprzewodnikowych i wykorzystaniem ich w bardziej skomplikowanych układach. W pierwszym przeglądzie nakreślono jak w obrębie jednej grupy związków - chalkohalogenków o stechiometrii MQX (gdzie: M = As, Sb, lub Bi; B = Se, S lub O; X = F, Cl, Br lub I) - zmiany polegające na wymianie atomu fluorowca lub stosunku atomów w cząsteczce wpływa na strukturę związku. W pracy tej zebrano informacje o strukturach różniących się między sobą jedynie atomem M, Q lub X, a także opisano cechy wspólne tych struktur i ich główne różnice. Dla części opisanych struktur możliwe okazało się też powiązanie tych niewielkich zmian strukturalnych ze zmianami w strukturze elektronowej. Drugi przegląd przedstawia między innymi obecny stan wiedzy na temat fotomemrystorów i układów wykazujących właściwości fotomemrystywne, ze szczególnym uwzględnieniem mechanizmu ich działania. W przeglądzie tym poruszono również tematykę obliczeń rezerwuarowych, zasady działania rezerwuarów, a także ich możliwe zastosowanie.
Quite briefly characterized hybrid organic-inorganic semiconductors are an interesting group of materials with potential applications in electronics, photocatalysis, photovoltaics, optoelectronics and related fields. The properties of molecular compounds depend profoundly on the weak intermolecular interaction. The strength of the interactions present in the crystal structure is reflected in the band structure and can be observed in the distribution of the density of the electronic states of individual materials. Strong interactions (covalent and ionic) in the inorganic semiconductors allow formation of continuous energy bands. On the other hand, much weaker van der Waals interactions, hydrogen bonds and stacking of the aromatic rings that occur in the purely organic semiconductors lead to narrower energy bands. In the case of hybrid semiconductors, both scenarios are possible and the predominance of weak or strong interactions in the structure determines the properties of the material. The hybrid semiconductors are represented in this dissertation by the tin-iodide complexes with organic ligands ([SnI4{(C6H5)3PO}2], [SnI4{(C6H5)2SO}2] and [SnI4(C5H5NO)2]). The iodine atoms present in these compounds are characterized by a large radius and polarizability which favors the formation of weaker bonds and intermolecular interactions. In these complexes, hydrogen interactions with iodine atoms combine the organic and inorganic fragments of the adjacent molecules. The weaker intermolecular interactions in the structure, the more discrete band structure and the more flat bands in the electronic structure. If these interactions are sufficiently weak, a border case may arise in which the material can be classified as an insulator with some of semiconductor features. This scenario in this work is represented by the organic triiodide salts. The crystal structures of these salts lack any strong intermolecular interactions and the linearly arranged I3 - anions are surrounded by large organic cations. Despite the weak interactions between the triiodide anions, orbitals of iodine atoms build both the edges of the valence and the conduction bands. Other weak interactions present in the structure encompass hydrogen interactions CH...I (distances H...I are usually too large to be called bonds). Despite their low charge carriers mobility, these materials efficiently generate photocurrents under the ultraviolet radiation due to the excitation of the electrons to the higher conduction bands that consist mostly of orbitals of the carbon atoms. The material based mainly on strong covalent bonds is lead (II) iodide - a semiconductor well described in the literature. Interestingly, PbI2 creates a Schottky junction with the surface of several metals. Surface states that lead to the formation of this barrier may have their sources in the chemisorption of a semiconductor molecules on the surface of the metal which may also cause the so-called "pillow effect" which reduces the work function of the metal. Another important source of surface states is the overlap of the electronic function of the metal with the electronic states (present in the energy gap of the semiconductor) created due to the finite dimensions of the crystals. An asymmetric filling and emptying of the surface states present at PbI2|Cu junction have been used to build a memristor - an element that changes its resistance as a result of the flow of the charge. In this work the analysis of the mechanism of operation of this device, the similarity of the constructed memristor to synapse and the use of a memristor for simple signal analysis are presented. The memristor was built into the reservoir system - a sub-type of the neural network - which allowed to perform simple calculations, e.g. on the classification of signals due to the amplitude. Another phenomenon observed in this system is metaplasticity. This type of neuromimetic behaviour manifests as the change of the basic properties of the device due to its history (i.e. flow of charges under prolonged positive bias). This effect is possible due to the formation of additional interaction between the metal and the semiconductor that changes the resistance of such joint. An example of this is an oxidation of the copper atoms that can subsequently form a bond with the iodine atoms present on the surface of PbI2. The operation of this entire system comes down to the interface interactions between the lead(II) halide and metal surface. This dissertation consists of five publications focusing on the physical chemistry of semiconductors and various aspects of information processing. Presented compounds differ significantly in composition and structure: 1) organic triiodide salts contain - in addition to the anion I3- large organic cations containing three or six aromatic rings; 2) tin(IV) iodide complexes have a molecular structure where the iodine atoms are bound by one covalent bond to a tin atom; 3) lead iodide(II) has a layered structure in which the iodine atom has the role of a bridge connecting neighbouring lead atoms. The diversity of the presented structures allows to investigate various interactions - strong covalent bonds, ionic bonds, weaker π-π interactions, hydrogen or halogen bonds and van der Waals interactions. The article that describes the properties of SnI4 complexes and triiodide salts focus on the impact of the individual structural elements on the electronic structure, e.g. conductivity, energy gap, mobility, type of doping and carrier concentration, and band edge potentials. In these publications it is shown that the weaker interactions present in the structure in large quantities can have a dominant influence on the electronic structure of the compound. The conclusions drawn are supported, among others, by spectroscopic measurements and theoretical calculations. Four compounds presented in this two articles: [(C6H5CH2)3NO]2H+I3-, (C6H5CH2)3NH+I3- ∙ C6H5CH3, [SnI4{(C6H5)2SO}2] and [SnI4(C5H5NO)2] are new with a structure that has never been described before. The third publication is focused on the device based on PbI2 and describes the interaction of this material with the surface of various metals (Au, Pt, Cu, Ag and Al). It also explains the effect of this interaction on the Schottky barrier responsible for the operation of this device. The dissertation is completed by two review papers that expands two topics taken in this work: the systematic analysis of the properties of the chalcohalogenides and the construction of the devices based on various semiconductors and their use in more complex systems. The first review outlines how in one group of compounds - chalcohalogenides with the stoichiometry MQX (where: M = As, Sb, or Bi; B = Se, S or O; X = F, Cl, Br or I) - the replacement of a halogen atom or the atomic ratio of atoms in the molecule affects the structure of the compound. In this work it was possible to gather information about structures differing only in M, Q or X atoms, to find the common features of these structures and their main differences. For some of the described structures, it was also possible to link these small structural differences with the changes in the electronic structure. The second review presents, among others, the current state of knowledge on photomemristors and systems exhibiting photomemristic properties, focusing on the mechanism of their operation. The review also touches on the subject of reservoir calculations, the principles of their work and their possible application.

Solvent effects play diverse roles in myriads of chemical, physical and biological processes. The solvent interacts with the solute by: i) non-specific (Coulombic, van der Waals interactions) and ii) specific interactions (hydrogen bonding, etc.). These interactions are responsible for solvation of the solute and are collectively termed as “solvent polarity”. Differential solvation of ground and excited electronic states is manifested in the absorption spectrum as a change in the band position, intensity or shape, which is termed as “solvatochromism”. Intermolecular hydrogen bonding (IHB) is a kind of specific solute-solvent interaction, which plays a key role in molecular or supramolecular photochemistry, as well as in photobiology. Solvation and its influence on various physico-chemical and biological processes can be understood by i) top-down; and ii) bottom-up approaches. In the top-down approach, the macroscopic properties like dielectric constant, refractive index are used to understand the microscopic solvation. This approach fails when specific interactions like hydrogen bonding interactions come into play, and furthermore it can reproduce only the macroscopic polarization of the solvent but fails miserably at the cybotactic region of solvation. With the recent advancements in the computational field, the molecular level description of solvation has been within reach for chemical physicists and experimentalists to corroborate their experimental results and in turn to visualize processes of fundamental or technologically relevant problems. The energy levels of the nπ* and ππ* singlet and triplet excited states of aromatic ketones are close-lying and therefore their energy levels can be altered by the substituents. The solvent polarity can be used as a surrogate to tune their energy levels. In certain cases, the lowest triplet or singlet excited states can switch their electronic character with increasing solvent polarity known as “electronic state switching” and thus modulate their photochemical or photophysical properties. Therefore, aromatic ketones were used as solvatochromic probes in this work. Comprehensive analyses of the solvent effects on xanthone (XT), 9,10-phenanthrenequinone (PQ) and benzophenone (Bzp) were carried out using steady-state and nanosecond time-resolved absorption, and resonance Raman spectroscopy in conjunction with ad hoc and classical-molecular dynamics and simulations generated supermolecule-continuum solvent model quantum mechanical calculations to corroborate the experimental outcomes and in turn to visualize the solvation process at the molecular level. The present thesis is divided into eight chapters and the summary of each chapter is described below: Chapter 1 provides a brief literature review of solvation effects and their influence on various physico-chemical and biological processes. Furthermore, the importance of understanding solvation at the molecular level and key concepts are discussed, which forms the heart of this thesis. Chapter 2 discusses the experimental and computational approaches used to study the solvation processes at the molecular level. A detailed explanation of spectroscopic techniques like resonance Raman (RR) and nanosecond-time resolved resonance Raman (ns-TR3) spectroscopy and their experimental and theoretical aspects are discussed, followed by a discussion on the fundamental concepts of computational methods like ab initio calculations density functional theory (DFT), and classical molecular dynamics and simulations (c-MDS) utilized in this study. Chapter 3 focuses on microscopic understanding of solvatochromic shifts observed for 9,10-phenanthrenequinone in protic solvents using UV-Vis and RR spectroscopy in conjunction with an ad hoc explicit solvation model and time-dependent density functional theory (TDDFT) calculations. The hypsochromic shift and bathochromic shift of the singlet nπ* and ππ* electronic transitions in protic solvents are due to hydrogen bond weakening and strengthening in the excited state for the corresponding electronic transitions, respectively as indicated by TD-DFT calculations and Kamlet-Taft linear solvation energy relationships. The hydrogen bond strengthening in the singlet ππ* excited state is further confirmed by Raman excitation profile (REP) analysis of PQ in different solvents. Furthermore, with increasing solvent polarity the two lowest singlet excited states undergo different hydrogen bonding mechanisms, leading to a decreasing energy gap between them. Therefore, hyperchromism of the nπ* transition has been hypothesized to be due to an increasing vibronic coupling between the lowest singlet nπ* and ππ* excited states. In Chapter 4, a real time observation of the thermal equilibrium between the lowest triplet excited states of PQ in acetonitrile solvent was carried out using ns-TR3 spectroscopy and this can explain its high reactivity towards H-atom abstraction, despite the fact that the lowest triplet excited state has ππ* character. Furthermore, extending the concept of hydrogen bonding mechanisms of the lowest singlet to the triplet excited states, the different hydrogen bonding mechanisms exhibited by them can lead to alteration of the intersystem crossing mechanisms in PQ. Chapter 5 highlights the very different role of intermolecular hydrogen bonding in the reduced reactivity of the xanthone (XT) triplet towards H-atom abstraction in protic solvents. The different hydrogen bonding mechanisms exhibited by the two lowest triplet excited states in protic solvents are derived from an ad hoc explicit solvation model, TD-DFT calculations and ns-time resolved absorption (ns-TRA): they separate them further in energy and thereby the nearest T2(nπ*) triplet state to the T1(ππ*) excited state plays an insignificant role in the reactivity towards H-atom abstraction, in contrast to the PQ triplet discussed in Chapter 4. Chapter 6 discusses the structure of XT triplet states using TR3 spectroscopy in combination with TD-DFT studies. The TR3 spectrum of the XT in acetonitrile identified a vibronic coupling mode responsible for the reactivity of XT towards H-atom abstraction, despite the fact that the lowest triplet excited state (T1) has ππ* character. This vibronic active mode is absent in the TR3 spectra of XT in protic solvents (methanol and ethanol). Furthermore, the REP analysis suggests that the nanosecond triplet-triplet absorption spectrum of XT in acetonitrile involves two different species, while in methanol it involves only one species. This observation is in agreement with the previous chapter (Chapter 5) which proposes a different hydrogen bonding mechanisms for the two lowest triplet excited states and their influence on the reduced reactivity towards H-atom abstraction. Chapters 3-6 emphasize the need for a proper solvation model at the molecular level to describe the various photophysical and photochemical processes of aromatic ketones. Therefore, Chapter 7 includes discussions on the bottom-up solvation methodology applied to benzophenone (Bzp) to understand its vibrational and electronic solvatochromic behaviour at the molecular level. Raman and UV-Vis spectroscopic techniques were used in conjunction with a c-MDS-generated supermolecule continuum solvation model DFT calculation to corroborate and to visualize the experimental outcome. The carbonyl stretching frequency of Bzp in protic solvents has two bands, corresponding to free carbonyl and hydrogen bonded carbonyl. Despite the fact that the macroscopic polarity of acetonitrile and methanol solvents are similar, the free carbonyl stretching of Bzp in methanol solvent was blue-shifted by 4 cm-1 with respect to the carbonyl stretching in acetonitrile solvent. The Gutmann’s acceptor number plot for carbonyl stretching frequencies indicates that the free carbonyl group is neighboured by a hydrophobic environment. The c-MDS-generated supermolecule-continuum solvation model DFT calculations suggest that the extended hydrogen bonding network of methanol solvent is responsible for the hydrophobic solvation around the free carbonyl. Furthermore, a linear correlation was obtained for the vibrational and electronic solvatochromism of the carbonyl frequency and energy of the singlet nπ* transition, respectively, which indicates that a variation in excitation wavelength for the singlet nπ* transition can arise from different solvation states. This can have implications for ultrafast processes associated with electron transfer, charge-transfer and also the photophysical aspects of excited states.Finally, Chapter 8 contains overall conclusions of the thesis and future directions for the present research area.

Solvent effects play diverse roles in myriads of chemical, physical and biological processes. The solvent interacts with the solute by: i) non-specific (Coulombic, van der Waals interactions) and ii) specific interactions (hydrogen bonding, etc.). These interactions are responsible for solvation of the solute and are collectively termed as “solvent polarity”. Differential solvation of ground and excited electronic states is manifested in the absorption spectrum as a change in the band position, intensity or shape, which is termed as “solvatochromism”. Intermolecular hydrogen bonding (IHB) is a kind of specific solute-solvent interaction, which plays a key role in molecular or supramolecular photochemistry, as well as in photobiology. Solvation and its influence on various physico-chemical and biological processes can be understood by i) top-down; and ii) bottom-up approaches. In the top-down approach, the macroscopic properties like dielectric constant, refractive index are used to understand the microscopic solvation. This approach fails when specific interactions like hydrogen bonding interactions come into play, and furthermore it can reproduce only the macroscopic polarization of the solvent but fails miserably at the cybotactic region of solvation. With the recent advancements in the computational field, the molecular level description of solvation has been within reach for chemical physicists and experimentalists to corroborate their experimental results and in turn to visualize processes of fundamental or technologically relevant problems. The energy levels of the nπ* and ππ* singlet and triplet excited states of aromatic ketones are close-lying and therefore their energy levels can be altered by the substituents. The solvent polarity can be used as a surrogate to tune their energy levels. In certain cases, the lowest triplet or singlet excited states can switch their electronic character with increasing solvent polarity known as “electronic state switching” and thus modulate their photochemical or photophysical properties. Therefore, aromatic ketones were used as solvatochromic probes in this work. Comprehensive analyses of the solvent effects on xanthone (XT), 9,10-phenanthrenequinone (PQ) and benzophenone (Bzp) were carried out using steady-state and nanosecond time-resolved absorption, and resonance Raman spectroscopy in conjunction with ad hoc and classical-molecular dynamics and simulations generated supermolecule-continuum solvent model quantum mechanical calculations to corroborate the experimental outcomes and in turn to visualize the solvation process at the molecular level. The present thesis is divided into eight chapters and the summary of each chapter is described below: Chapter 1 provides a brief literature review of solvation effects and their influence on various physico-chemical and biological processes. Furthermore, the importance of understanding solvation at the molecular level and key concepts are discussed, which forms the heart of this thesis. Chapter 2 discusses the experimental and computational approaches used to study the solvation processes at the molecular level. A detailed explanation of spectroscopic techniques like resonance Raman (RR) and nanosecond-time resolved resonance Raman (ns-TR3) spectroscopy and their experimental and theoretical aspects are discussed, followed by a discussion on the fundamental concepts of computational methods like ab initio calculations density functional theory (DFT), and classical molecular dynamics and simulations (c-MDS) utilized in this study. Chapter 3 focuses on microscopic understanding of solvatochromic shifts observed for 9,10-phenanthrenequinone in protic solvents using UV-Vis and RR spectroscopy in conjunction with an ad hoc explicit solvation model and time-dependent density functional theory (TDDFT) calculations. The hypsochromic shift and bathochromic shift of the singlet nπ* and ππ* electronic transitions in protic solvents are due to hydrogen bond weakening and strengthening in the excited state for the corresponding electronic transitions, respectively as indicated by TD-DFT calculations and Kamlet-Taft linear solvation energy relationships. The hydrogen bond strengthening in the singlet ππ* excited state is further confirmed by Raman excitation profile (REP) analysis of PQ in different solvents. Furthermore, with increasing solvent polarity the two lowest singlet excited states undergo different hydrogen bonding mechanisms, leading to a decreasing energy gap between them. Therefore, hyperchromism of the nπ* transition has been hypothesized to be due to an increasing vibronic coupling between the lowest singlet nπ* and ππ* excited states. In Chapter 4, a real time observation of the thermal equilibrium between the lowest triplet excited states of PQ in acetonitrile solvent was carried out using ns-TR3 spectroscopy and this can explain its high reactivity towards H-atom abstraction, despite the fact that the lowest triplet excited state has ππ* character. Furthermore, extending the concept of hydrogen bonding mechanisms of the lowest singlet to the triplet excited states, the different hydrogen bonding mechanisms exhibited by them can lead to alteration of the intersystem crossing mechanisms in PQ. Chapter 5 highlights the very different role of intermolecular hydrogen bonding in the reduced reactivity of the xanthone (XT) triplet towards H-atom abstraction in protic solvents. The different hydrogen bonding mechanisms exhibited by the two lowest triplet excited states in protic solvents are derived from an ad hoc explicit solvation model, TD-DFT calculations and ns-time resolved absorption (ns-TRA): they separate them further in energy and thereby the nearest T2(nπ*) triplet state to the T1(ππ*) excited state plays an insignificant role in the reactivity towards H-atom abstraction, in contrast to the PQ triplet discussed in Chapter 4. Chapter 6 discusses the structure of XT triplet states using TR3 spectroscopy in combination with TD-DFT studies. The TR3 spectrum of the XT in acetonitrile identified a vibronic coupling mode responsible for the reactivity of XT towards H-atom abstraction, despite the fact that the lowest triplet excited state (T1) has ππ* character. This vibronic active mode is absent in the TR3 spectra of XT in protic solvents (methanol and ethanol). Furthermore, the REP analysis suggests that the nanosecond triplet-triplet absorption spectrum of XT in acetonitrile involves two different species, while in methanol it involves only one species. This observation is in agreement with the previous chapter (Chapter 5) which proposes a different hydrogen bonding mechanisms for the two lowest triplet excited states and their influence on the reduced reactivity towards H-atom abstraction. Chapters 3-6 emphasize the need for a proper solvation model at the molecular level to describe the various photophysical and photochemical processes of aromatic ketones. Therefore, Chapter 7 includes discussions on the bottom-up solvation methodology applied to benzophenone (Bzp) to understand its vibrational and electronic solvatochromic behaviour at the molecular level. Raman and UV-Vis spectroscopic techniques were used in conjunction with a c-MDS-generated supermolecule continuum solvation model DFT calculation to corroborate and to visualize the experimental outcome. The carbonyl stretching frequency of Bzp in protic solvents has two bands, corresponding to free carbonyl and hydrogen bonded carbonyl. Despite the fact that the macroscopic polarity of acetonitrile and methanol solvents are similar, the free carbonyl stretching of Bzp in methanol solvent was blue-shifted by 4 cm-1 with respect to the carbonyl stretching in acetonitrile solvent. The Gutmann’s acceptor number plot for carbonyl stretching frequencies indicates that the free carbonyl group is neighboured by a hydrophobic environment. The c-MDS-generated supermolecule-continuum solvation model DFT calculations suggest that the extended hydrogen bonding network of methanol solvent is responsible for the hydrophobic solvation around the free carbonyl. Furthermore, a linear correlation was obtained for the vibrational and electronic solvatochromism of the carbonyl frequency and energy of the singlet nπ* transition, respectively, which indicates that a variation in excitation wavelength for the singlet nπ* transition can arise from different solvation states. This can have implications for ultrafast processes associated with electron transfer, charge-transfer and also the photophysical aspects of excited states.Finally, Chapter 8 contains overall conclusions of the thesis and future directions for the present research area.

The thesis entitled “Organic fluorine in crystal engineering: Consequences on molecular and supramolecular organization” consists of six chapters. The main theme of the thesis is to address the role of substituted fluorine atoms in altering the geometrical and electronic features in organic molecules and its subsequent consequences on crystal packing. The thesis is divided into three parts. Part I deals with compounds that are liquids under ambient conditions, crystal structures of which have been determined by the technique of in situ cryocrystallography. Part II demonstrates the utilization of in situ cryocrystallography to study kinetically trapped metastable crystalline phases that provide information about crystallization pathways. In part III, crystal structures of a series of conformationally flexible molecules are studied to evaluate the consequences of fluorine substitution on the overall molecular conformation. The genesis and stabilization of a particular molecular conformation has been rationalized in terms of variability in intermolecular interactions in the crystalline state. Part I. In situ cryocrystallography: Probing the solid state structures of ambient condition liquids. Chapter 1 discusses the crystal structures of benzoyl chloride and its fluorinated analogs. These compounds have been analysed for the propensity of adoption of Cl···O halogen bonded dimers and catemers. The influence of conformational and electronic effects of sequential fluorination on the periphery of the phenyl ring has been quantified in terms of the most positive electrostatic potential, VS,max (corresponding to σ-hole) on the Cl-atom. It is shown that fluorine also exhibits “amphoteric” nature like other heavier halogens, particularly in presence of electron withdrawing groups. Although almost all the derivatives pack through C–H···O, C–H···F, C–H···Cl, Cl···F, C–H···π and π···π interactions, the compound 2,3,5,6-tetrafluorobenzoyl chloride exhibited a not so commonly observed Cl···O halogen bonded catemer. On the other hand, the proposed Cl···O mediated dimer is not observed in any of the structures due to geometrical constraints in the crystal lattice. Chapter 2 presents the preferences of fluorine to form hydrogen bond (C–H···F) and halogen bonds (X···F; X= Cl, Br, I). Crystal structures of all three isomers of chloro-, bromo-and iodo-fluorobenzene have been probed in order to gain insights into packing interactions preferred by fluorine and other heavier halogens. It has been observed that homo halogen…halogen (Cl···Cl, Br···Br and I···I) contacts prevail in most of the structures with fluorine being associated with the hydrogen atom forming C–H···F hydrogen bond. The competition between homo and hetero halogen bonds (I···I vs I···F) is evident from the packing polymorphism exhibited by 4-iodo fluorobenzene observed under different cooling protocols. The crystal structures of pentafluoro halo (Cl, Br, I) benzenes were also determined in order to explore the propensity of formation of homo halogen bonds over hetero halogen bonds. Different dimeric and catemeric motifs based on X···F and F···F interactions were observed in these structures. Chapter 3 focuses on the effect of different cooling protocols in generating newer polymorphs of a given liquid. The third polymorph (C2/c, Z'=6) of phenylacetylene was obtained by sudden quenching of the liquid filled in capillary from a hot water bath (363 K) to the nitrogen bath (< 77 K). Also, different polymorphs were obtained for both 2¬fluoro phenylacetylene (Pna21, Z'=1) and 3-fluoro phenylacetylene (P21/c, Z'=3) when crystallized by sudden quenching in contrast to the generally followed method of slow cooling which results in isostructural forms (P21, Z'=1). The rationale for these kinetically stable “arrested” crystalline configurations is provided in part II of the thesis. Part II. Tracing crystallization pathways via kinetically captured metastable forms. Chapter 4 explains the utilization of the new approach of sudden quenching of liquids (detailed in chapter 3) to obtain kinetically stable (metastable) crystalline phases that appear to be closer to the unstructured liquids. Six different examples namely, phenylacetylene, 2-fluorophenylacetylene, 3-fluorophenylacetylene, 4-fluorobenzoyl chloride, 3-chloro fluorobenzene and ethyl chloroformate are discussed in this context. In each case, different polymorphs were obtained when the liquid was cooled slowly (100 K/h) and when quenched sharply in liquid nitrogen. The relationship between these metastable forms and the stable forms (obtained by slow cooling) combined with the mechanistic details of growth of stable forms from metastable forms provides clues about the crystallization pathways. Part III. Conformational analysis in the solid state: Counterbalance of intermolecular interactions with molecular and crystallographic symmetries. Chapter 5 describes the crystal structures of a series of conformationally flexible molecules namely, acetylene and diacetylene spaced aryl biscarbonates and biscarbamates. While most of the molecules adopt commonly anticipated anti (transoid) conformation, some adopt unusual cisoid and gauche conformations. It is shown that the unusually twisted conformation of one of the compounds [but-2-yne-bis(2,3,4,5,6¬pentafluorocarbonate)] is stabilized mainly by the extraordinarily short C–H···F intermolecular hydrogen bond. The strength of this rather short C–H···F hydrogen bond has been authenticated by combined single crystal neutron diffraction and X-ray charge density analysis. It has also been shown that the equi-volume relationship of H-and F-atoms (H/F isosterism) can be explored to access various possible conformers of a diacetylene spaced aryl biscarbonate. While biscarbonates show variety of molecular conformations due to absence of robust intermolecular interactions, all the biscarbamates adopt anti conformation where the molecules are linked with antiparallel chains formed with N–H···O=C hydrogen bonds. Chapter 6 presents a unique example where the commonly encountered crystallographic terms namely, high Z' structure, polymorphism, phase transformation, disorder, isosterism and isostructuralism are witnessed in a single molecular species (parent compound benzoylcarvacryl thiourea and its fluorine substituted analogs). The origin of all these phenomenon has been attributed to the propensity of formation of a planar molecular dimeric chain mediated via N–H···O [R2 (12)] and N–H···S [R2 (8)] dimers.

The thesis entitled “Organic fluorine in crystal engineering: Consequences on molecular and supramolecular organization” consists of six chapters. The main theme of the thesis is to address the role of substituted fluorine atoms in altering the geometrical and electronic features in organic molecules and its subsequent consequences on crystal packing. The thesis is divided into three parts. Part I deals with compounds that are liquids under ambient conditions, crystal structures of which have been determined by the technique of in situ cryocrystallography. Part II demonstrates the utilization of in situ cryocrystallography to study kinetically trapped metastable crystalline phases that provide information about crystallization pathways. In part III, crystal structures of a series of conformationally flexible molecules are studied to evaluate the consequences of fluorine substitution on the overall molecular conformation. The genesis and stabilization of a particular molecular conformation has been rationalized in terms of variability in intermolecular interactions in the crystalline state. Part I. In situ cryocrystallography: Probing the solid state structures of ambient condition liquids. Chapter 1 discusses the crystal structures of benzoyl chloride and its fluorinated analogs. These compounds have been analysed for the propensity of adoption of Cl···O halogen bonded dimers and catemers. The influence of conformational and electronic effects of sequential fluorination on the periphery of the phenyl ring has been quantified in terms of the most positive electrostatic potential, VS,max (corresponding to σ-hole) on the Cl-atom. It is shown that fluorine also exhibits “amphoteric” nature like other heavier halogens, particularly in presence of electron withdrawing groups. Although almost all the derivatives pack through C–H···O, C–H···F, C–H···Cl, Cl···F, C–H···π and π···π interactions, the compound 2,3,5,6-tetrafluorobenzoyl chloride exhibited a not so commonly observed Cl···O halogen bonded catemer. On the other hand, the proposed Cl···O mediated dimer is not observed in any of the structures due to geometrical constraints in the crystal lattice. Chapter 2 presents the preferences of fluorine to form hydrogen bond (C–H···F) and halogen bonds (X···F; X= Cl, Br, I). Crystal structures of all three isomers of chloro-, bromo-and iodo-fluorobenzene have been probed in order to gain insights into packing interactions preferred by fluorine and other heavier halogens. It has been observed that homo halogen…halogen (Cl···Cl, Br···Br and I···I) contacts prevail in most of the structures with fluorine being associated with the hydrogen atom forming C–H···F hydrogen bond. The competition between homo and hetero halogen bonds (I···I vs I···F) is evident from the packing polymorphism exhibited by 4-iodo fluorobenzene observed under different cooling protocols. The crystal structures of pentafluoro halo (Cl, Br, I) benzenes were also determined in order to explore the propensity of formation of homo halogen bonds over hetero halogen bonds. Different dimeric and catemeric motifs based on X···F and F···F interactions were observed in these structures. Chapter 3 focuses on the effect of different cooling protocols in generating newer polymorphs of a given liquid. The third polymorph (C2/c, Z'=6) of phenylacetylene was obtained by sudden quenching of the liquid filled in capillary from a hot water bath (363 K) to the nitrogen bath (< 77 K). Also, different polymorphs were obtained for both 2¬fluoro phenylacetylene (Pna21, Z'=1) and 3-fluoro phenylacetylene (P21/c, Z'=3) when crystallized by sudden quenching in contrast to the generally followed method of slow cooling which results in isostructural forms (P21, Z'=1). The rationale for these kinetically stable “arrested” crystalline configurations is provided in part II of the thesis. Part II. Tracing crystallization pathways via kinetically captured metastable forms. Chapter 4 explains the utilization of the new approach of sudden quenching of liquids (detailed in chapter 3) to obtain kinetically stable (metastable) crystalline phases that appear to be closer to the unstructured liquids. Six different examples namely, phenylacetylene, 2-fluorophenylacetylene, 3-fluorophenylacetylene, 4-fluorobenzoyl chloride, 3-chloro fluorobenzene and ethyl chloroformate are discussed in this context. In each case, different polymorphs were obtained when the liquid was cooled slowly (100 K/h) and when quenched sharply in liquid nitrogen. The relationship between these metastable forms and the stable forms (obtained by slow cooling) combined with the mechanistic details of growth of stable forms from metastable forms provides clues about the crystallization pathways. Part III. Conformational analysis in the solid state: Counterbalance of intermolecular interactions with molecular and crystallographic symmetries. Chapter 5 describes the crystal structures of a series of conformationally flexible molecules namely, acetylene and diacetylene spaced aryl biscarbonates and biscarbamates. While most of the molecules adopt commonly anticipated anti (transoid) conformation, some adopt unusual cisoid and gauche conformations. It is shown that the unusually twisted conformation of one of the compounds [but-2-yne-bis(2,3,4,5,6¬pentafluorocarbonate)] is stabilized mainly by the extraordinarily short C–H···F intermolecular hydrogen bond. The strength of this rather short C–H···F hydrogen bond has been authenticated by combined single crystal neutron diffraction and X-ray charge density analysis. It has also been shown that the equi-volume relationship of H-and F-atoms (H/F isosterism) can be explored to access various possible conformers of a diacetylene spaced aryl biscarbonate. While biscarbonates show variety of molecular conformations due to absence of robust intermolecular interactions, all the biscarbamates adopt anti conformation where the molecules are linked with antiparallel chains formed with N–H···O=C hydrogen bonds. Chapter 6 presents a unique example where the commonly encountered crystallographic terms namely, high Z' structure, polymorphism, phase transformation, disorder, isosterism and isostructuralism are witnessed in a single molecular species (parent compound benzoylcarvacryl thiourea and its fluorine substituted analogs). The origin of all these phenomenon has been attributed to the propensity of formation of a planar molecular dimeric chain mediated via N–H···O [R2 (12)] and N–H···S [R2 (8)] dimers.

Work reported in this thesis mainly comprises of the assignments and analysis of the rotational spectra and structures of three weakly bound complexes: C2H4•••H2S, C6H5CCH•••H2O and C6H5CCH•••H2S. All the data have been collected using a home built Pulsed Nozzle Fourier Transform Microwave Spectrometer. Apart from this, the thesis also deals with a criterion of classifying a weakly bound complex to a ‘hydrogen-bonded’ one. First chapter of the thesis gives a brief intermolecular interactions and molecular clusters of π system. It also briefly touches on the structural determination by rotational spectroscopy and the basic information one can gain from the rotational spectrum. Second chapter of the thesis gives a brief introduction to the experimental and theoretical methodology. It also gives a description of the software used in the FTMW spectrometer which was rebuilt using Labview 7.1. Third chapter of the thesis deals with the rotational spectra and structure of eight isotopologoues of C2H4•••H2S complex. The lines are split into four components for the parent isotopologue due to the presence of large amplitude motion. The smaller splitting is 0.14 MHz and the higher splitting is 1.67 MHz in (B+C)/2 for the parent isotopologue. Spectral splitting pattern of the isotopologues confirmed that smaller splitting is due to the rotation of ethylene about its C-C bond axis along with the contraction of S-H bond whereas the larger motion arises due to the interchange of equivalent hydrogens of H2S in the complex. A detailed spectral analysis and ab initio calculation for this system have been described in chapter III. The fourth chapter of the thesis describes the rotational spectroscopic studies of five isotopologues of C6H5CCH•••H2O complex. Rotational spectra unequivocally confirm the structure of the complex to be a one where H2O is donating one of its hydrogen to the acetylenic π cloud forming a O-H••• π bond whereas the ring ortho C-H bond forms C-H•••O bond with the water oxygen. For theparent isotopomer the lines are split into two components due to the rotation of H2O about its C2 symmetric axis. The fifth chapter of thesis describes the rotational spectroscopic and ab initio studies of five isotopologues of C6H5CCH•••H2S complex. Rotational spectra indicate the structure to be the one where H2S is sitting on the top of the phenyl ring and shifted towards the acetylenic group. The sixth chapter of the thesis describes a criterion for calling a complex to be hydrogen bonded based on the dynamic structure rather than the static structure of the complex. The question asked is if the anisotropy of the interaction is strong enough to hold the ‘hydrogen bond’ when one takes dynamics into account. The proposed criterion is that the zero point energy of the motion which takes the hydrogen away from the acceptor should be much less than the barrier height of the respective motion supporting at least one bound level below the barrier. Ab initio calculations have been done on four model systems Ar2•••H2O, Ar2•••H2S, C2H4••• H2O and C2H4••• H2S to emphasize this criterion.

Work reported in this thesis mainly comprises of the assignments and analysis of the rotational spectra and structures of three weakly bound complexes: C2H4•••H2S, C6H5CCH•••H2O and C6H5CCH•••H2S. All the data have been collected using a home built Pulsed Nozzle Fourier Transform Microwave Spectrometer. Apart from this, the thesis also deals with a criterion of classifying a weakly bound complex to a ‘hydrogen-bonded’ one. First chapter of the thesis gives a brief intermolecular interactions and molecular clusters of π system. It also briefly touches on the structural determination by rotational spectroscopy and the basic information one can gain from the rotational spectrum. Second chapter of the thesis gives a brief introduction to the experimental and theoretical methodology. It also gives a description of the software used in the FTMW spectrometer which was rebuilt using Labview 7.1. Third chapter of the thesis deals with the rotational spectra and structure of eight isotopologoues of C2H4•••H2S complex. The lines are split into four components for the parent isotopologue due to the presence of large amplitude motion. The smaller splitting is 0.14 MHz and the higher splitting is 1.67 MHz in (B+C)/2 for the parent isotopologue. Spectral splitting pattern of the isotopologues confirmed that smaller splitting is due to the rotation of ethylene about its C-C bond axis along with the contraction of S-H bond whereas the larger motion arises due to the interchange of equivalent hydrogens of H2S in the complex. A detailed spectral analysis and ab initio calculation for this system have been described in chapter III. The fourth chapter of the thesis describes the rotational spectroscopic studies of five isotopologues of C6H5CCH•••H2O complex. Rotational spectra unequivocally confirm the structure of the complex to be a one where H2O is donating one of its hydrogen to the acetylenic π cloud forming a O-H••• π bond whereas the ring ortho C-H bond forms C-H•••O bond with the water oxygen. For theparent isotopomer the lines are split into two components due to the rotation of H2O about its C2 symmetric axis. The fifth chapter of thesis describes the rotational spectroscopic and ab initio studies of five isotopologues of C6H5CCH•••H2S complex. Rotational spectra indicate the structure to be the one where H2S is sitting on the top of the phenyl ring and shifted towards the acetylenic group. The sixth chapter of the thesis describes a criterion for calling a complex to be hydrogen bonded based on the dynamic structure rather than the static structure of the complex. The question asked is if the anisotropy of the interaction is strong enough to hold the ‘hydrogen bond’ when one takes dynamics into account. The proposed criterion is that the zero point energy of the motion which takes the hydrogen away from the acceptor should be much less than the barrier height of the respective motion supporting at least one bound level below the barrier. Ab initio calculations have been done on four model systems Ar2•••H2O, Ar2•••H2S, C2H4••• H2O and C2H4••• H2S to emphasize this criterion.