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1
Wilkinson, David Adam. "Molecular hydrogen in galaxies". Thesis, Durham University, 1987. http://etheses.dur.ac.uk/6657/.
Testo completoAbstract (sommario):
This study aims to understand the key role played by molecular hydrogen in the evolution of galaxies, with a view to constraining its radial distribution in the Galaxy and the CO→H(_2) conversion factor α(_20).The star formation rate is shown to be correlated with the surface density of H(_2). A correlation between the molecular hydrogen fraction and the metallicity of a region allows the time evolution of H(_2) to be described. This leads to a modified 'Schmidt Law' of the SFR which explains quite naturally the production of galactic metallicity gradients and the constancy of the SFR in the absence of infall. A consistent closed model of the chemical evolution of the Galaxy is proposed to solve the G-dwarf problem, the stellar age-metallicity relation and the metallicity gradient, leading to the prediction of some initial amount of pre-disc processing of gas into visible and dark matter. It is found that a constant yield of metals is more appropriate than a yield proportional to metallicity. Possible time variations of the returned fraction, the dark matter fraction and the SFR are also studied. For consistency, we suggest that dark matter in the solar neighbourhood could be totally baryonic provided the Miller-Scalo IMF is modified at the lower end, that is, the dark matter resides in low mass stars or brown dwarfs. The production of metallicity gradients in spiral galaxies is shown to be a direct consequence of the radial variation of the total surface density of matter and the age of the disc. The role of molecular gas in the evolution of the Oort Cloud of comets is examined. It is shown that comet showers with a mean interval of ̴̱ 30My cannot be produced using perturbations of the Oort Cloud by known stars or molecular clouds. If there is indeed an apparent 30My periodicity in the terrestrial mass extinction and geological records, we argue that astronomically induced processes are unlikely to be the primary cause. Evidence is presented that the lifetime of the molecular gas phase is ≤ 2.lO(_8)y, and arguments, particularly from CO observations of the Virgo galaxy cluster, favouring longer lifetimes are shown to be not well founded. We suggest that the ICM in Virgo reduces the value of α(_20) as compared to isolated galaxies. From the above considerations, the radial distribution of in the Galaxy is derived and shown to agree in the inner Galaxy with that derived from ɤ-ray analysis. In the solar neighbourhood we find α(_20) = 2.5±0.5, and present evidence that α(_20) varies as a function of Galactocentric radius and from galaxy to galaxy.
2
Li, Chengguang. "Molecular hydrogen in planetary nebulae". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0027/MQ31359.pdf.
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3
Gatti, Francesco Gilberto. "Hydrogen bond-assembled molecular shuttles". Thesis, University of Warwick, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.247443.
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4
Pérez, Emilio M. "Hydrogen-bonded synthetic molecular machines". Thesis, University of Edinburgh, 2004. http://hdl.handle.net/1842/15610.
Testo completoAbstract (sommario):
This thesis reports on: 1) the development of two new methods to provoke the translation of the macrocycle along the thread (“shuttling”) in hydrogen bonded, fumaramide-based [2]rotaxanes and 2) the utilization of that movement to provoke a potentially useful response. The fumaramide template is perfectly preorganised to form four intercomponent hydrogen bonds with a benzylic amide macrocycle, affording [2]rotaxanes in “world record” yields. This preorganisation can be disrupted by photo-isomerisation (254 nm) of the E double bond to its Z counterpart. The newly formed maleamide template shows little affinity for the macrocycle. This has previously been exploited to synthesise a light and heat switchable molecular shuttles. A unique tristable molecular shuttle in which the macrocycle can be located in three different “stations” by means of thermal and photochemical stimuli is described in Chapter Two. In Chapter Three an alternative mechanism of shuttling for fumaramide-based molecular shuttles is reported. The reversibility of Diels-Alder chemistry is exploited to synthesise a chemically driven molecular shuttle. A chiral two-station [2]rotaxane in which translational motion of the macrocycle along the thread results in a profound change in its optical properties (CD spectrum) is described in Chapter Four. Finally, a light-switchable optically-addressable molecular shuttle is discussed. A [2]rotaxane with a thread containing a fluorophore and a macrocycle functionalised to quench its fluorescence was synthesised. Shuttling of the macrocycle along the thread switched the fluorescence “on” and “off”.
5
Onay, Aytun. "Hydrogen Storage Capacity Of Nanosystems: Molecular". Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/3/12609636/index.pdf.
Testo completoAbstract (sommario):
In recent decades, tremendous efforts have been made to obtain high hydrogen storage capacity in a stable configuration. In the literature there are plenty of experimental works investigating different materials for hydrogen storage and their storage values. In the first part of this thesis the available literature data have been collected and tabulated. In addition to the literature survey the hydrogen storage capacity of carbon nanotubes and carbon nanotubes doped with boron nitride (CBN nanotubes) with different chirality have been investigated by performing quantum chemical methods at semiempirical and DFT levels of calculations. It has been found that boron nitrite doping increases the hydrogen storage capacity of carbon nanotubes. Single wall carbon nanotubes (SWNT) can be thought as formed by warping a single graphitic layer into a cylindrical object. SWNTs attract much attention because they have unique electronic properties,
very strong structure and high elastic moduli. The systems under study include the structures C(4,4), H2@C(4,4), C(7,0), C(4,0), and the BN doped C(4,4), H2@C(4,4), 2H2@C(4,4), C(7,0), H2@C(7,0), 2H2@C(7,0). Also, we have investigated adsorption and desorption of hydrogen molecules on BN doped coronene models by means of theoretical calculations.
6
Montgomerie, Christine Ann. "Spectroscopy of the hydrogen molecular ion". Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257936.
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7
Guest, Michael Arthur. "The infrared spectrum of molecular hydrogen". Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315022.
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8
Ray, Mark D. "Precision Lifetime Measurements in Molecular Hydrogen /". The Ohio State University, 1995. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487931512617771.
Testo completo
9
Chen, Guo Fu. "The diffusion of muonic hydrogen atoms in hydrogen gas". W&M ScholarWorks, 1990. https://scholarworks.wm.edu/etd/1539623790.
Testo completoAbstract (sommario):
This experiment measured the time distribution of muonic hydrogen atoms which were formed when negative muons were brought to rest in H{dollar}\sb2{dollar} gas, containing Au target foils, at five pressures (750 mbar, 375 mbar, 188 mbar, 94 mbar and 47 mbar at 4.6 mm foil spacing). A Monte Carlo method is applied for deducing the initial velocity distribution, and preliminary results are obtained. The initial velocity distribution of {dollar}\mu{dollar}H atoms is reasonably well described as a 'Maxwellian' velocity distribution with a mean energy E = 3.4 eV. The corresponding muon mean capture energy is obtained: E{dollar}\sb{lcub}\rm c{rcub}{dollar} {dollar}\approx{dollar} 34 eV for {dollar}\mu{dollar}H atom and E{dollar}\sb{lcub}\rm c{rcub}{dollar} {dollar}\approx{dollar} 68 eV for {dollar}\mu{dollar}H{dollar}\sb2{dollar} molecules. We also find the negative muon capture energy distribution is exponential.;In addition, a significant improvement of the negative muon mean life {dollar}\tau{dollar} in Au is abtained in this experiment.: {dollar}\tau\sb{lcub}\rm Au{rcub}{dollar} = 69.716 {dollar}\pm{dollar} 0.144 ns. The "full decay curve fitting method" which we use in this experiment has an advantage over the previous method in three aspects: (1) We have measured the mean life and determined the time resolution {dollar}\sigma{dollar}(E) of a detector at a particular energy level; (2) We have determined the effective zero time of the decay curve; (3) We have provided a possible way to measure the mean life {dollar}\tau{dollar} when {dollar}\tau{dollar} is less than the time resolution {dollar}\sigma{dollar}(E) of the detector ({dollar}\tau{dollar} {dollar}<{dollar} {dollar}\sigma{dollar}(E)).
10
Chrysostomou, Antonio. "Molecular hydrogen line emission from photodissociation regions". Thesis, University of Edinburgh, 1993. http://hdl.handle.net/1842/27794.
Testo completoAbstract (sommario):
The work presented in this thesis is dedicated to the study of the physical properties of photodissociation regions (PDRs), the surface layers of molecular clouds which are irradiated by ultraviolet radiation. The structure of PDRs is investigated with the development of an analytical model which incorporates the essential heating and cooling mechanisms in a PDR. The main parameters in the model are the density and the incident ultraviolet radiation field, above the ambient value in the solar neighbourhood, impinging on the surface (G0) which dissociates the molecules in the PDR. It is demonstrated that when the ratio (n/G0) is high (> 100 cm-3) the attenuation of ultraviolet photons is dominated by H2 self shielding which brings the HI/H2 transition zone close to the surface of the cloud (Av < 1). When the ratio is of order unity then the attenuation of ultraviolet photons is dominated by dust grains in the PDR. In this case, the HI/H2 transition zone occurs at a depth of Av ~ 2 - 3. Images of the PDR in the northern bar of M17 show that there is a spatial coincidence, accurate to ~ 1 arcsec, of the H2 and 3.28 μm emission regions (the 3.28 μm emission being a tracer of the hot edge of the PDR delineated by the HII/HI transition) placing a lower limit to the density in the clumps of 105 cm-3. This coincidence is also observed in other PDR sources (eg. NGC 2023) and can be readily explained if the sources are clumpy. It is not clear in the northern bar of M17, where G0 ~ 10^4, whether shielding by dust or H_2 molecules is dominating the attenuation of ultraviolet photons. A uniform, high density PDR model is sufficient to reproduce the observed H_2 line intensity, however the images clearly reveal structures at the 2 arcsec level suggesting that a clumpy model is a realistic solution.
11
Tedds, Jonathan Andrew. "Shocked molecular hydrogen in the Orion "bullets"". Thesis, University of Edinburgh, 1997. http://hdl.handle.net/1842/27519.
Testo completoAbstract (sommario):
The physics of shocked outflows in molecular clouds is one of the fundamental astrophysical processes by which the cycle of star formation in our Galaxy is regulated. I outline the basis of our understanding of the star formation process and the violent outflow always associated with it, the physics of shocks in molecular gas, and the consequent excitation of molecular hydrogen (H2). It is demonstrated that molecular hydrogen is the best observational diagnostic of this hot, shocked molecular gas and an introduction is given to the observational techniques of near-infrared spectroscopy required in its measurement. I describe a detailed observational study of the physics of shocked H2 excitation and dynamics in the nearby massive star forming region of the Orion giant molecular cloud, the brightest source of its type, using the recently upgraded CGS4 near-IR spectrometer at UKIRT. We have demonstrated that integrated [FeII] 1.644μm line profiles in the Orion "bullets" are consistent with theoretical bow-shock predictions for two different "bullets". We have identified a uniform, broad background component pervading the region in both Fe+ and H2 which is inconsistent with a fluorescent component due to the ionizing radiation of the Trapezium stars alone. A collisionally broadened background component of unidentified origin is measured to be Gaussian in profile with an average FWHM of 26±2.5kms-1 in the H2 1-0 S(1) line after deconvolution of the instrument profile and a peak velocity of 2.5±0.5kms-1, close to the local ambient rest velocity. Crucially, the extended H2 "bullet" wakes have allowed us to dissect individual molecular bow shock structures but the broad (intrinsic FWHM≤27mks-1), singly-peaked H2 1-0 S(1) profiles observed in the two most clearly resolved, plane-of-sky oriented wakes challenge our present understanding. It is very difficult to reconcile any steady-state molecular bow shock model with these observations in Orion. To fit a single C shock absorber model to individual H2 profiles implies a magnetic field strength far in excess of observed estimates and is not consistent with the bow-shaped wake morphology.
12
Theobald, James Andrew. "Self-assembly of hydrogen-bonded molecular traps". Thesis, University of Nottingham, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.416730.
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13
Gerst, Steven Gregory. "The deuterium content of atmospheric molecular hydrogen /". Thesis, Connect to this title online; UW restricted, 2000. http://hdl.handle.net/1773/11551.
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14
Undurti, Sainadh. "`Attoclock' experiments on atomic and molecular hydrogen". Thesis, Griffith University, 2018. http://hdl.handle.net/10072/381373.
Testo completoAbstract (sommario):
The current thesis aims at benchmarking strong- eld physics with the help of precision
measurements performed on the simplest atomic (H) and molecular (H2) systems.
The importance of H in validating strong- eld models is demonstrated through
the rst set of experimental data. It aims at calibrating the absolute Carrier-envelope
phase (CEP) of few-cycle laser pulses using H against complete ab initio solution of
the three dimensional time-dependent Schr odinger equation (3D-TDSE). Subsequent
set of measurements with noble gases against widely used strong- eld models based
on single-active electron (SAE) approximation, is shown to reveal a systematic o set
of 0:25 radians in tagging CEP, questioning the validity of such models.
The second experimental study forms the main result of this thesis, that attempts
to resolve the ongoing debate on tunnelling times (tunnelling delays in the context of
strong- eld physics). We address this by employing the `attoclock' technique with 6
fs pulses on H and validating the results against full numerical solutions of ab initio
3D-TDSE. The validated numerical codes are then used to arti cially screen the
parent ion-electron interaction, concluding that the tunnelling time 1.8 as.
The nal experimental results presented in this dissertation are the alignmentdependent
attoclock measurements using both few-cycle (7 fs) and multi-cycle (28 fs)
pulses on H2. The measured attoclock observable for various molecular orientations
(in laser polarisation frame) shows a strong modulation with a periodicity of .
Initial ab initio simulations for few-cycle pulses under the frozen-nuclei and SAE
approximations, fail to explain these observations. Further experimental studies with
H2/D2 (50:50 mixed gases) show no signi cant relative di erences among the attoclock
observables, suggesting a prominent role of the electron-electron correlations at play.
The ongoing study is believed to have far reaching implications in applications such
as studying molecular dissociation processes and tomography.Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text
15
Pejlovas, Aaron Matthew. "Microwave Spectra and Molecular Structures of Organic Molecules and Hydrogen Bonded Dimers". Thesis, The University of Arizona, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10748431.
Testo completoAbstract (sommario):
The microwave spectra were measured in the 4–15 GHz regime for cyclopropanecarboxylic acid, 1,2-cyclohexanedione, maleimide, phthalimide, and 4a,8a-azaboranaphthalene. Doubly hydrogen bonded dimers formed with formic acid were also measured with the molecules cyclopropanecarboxylic acid, 1,2-cyclohexanedione, maleimide, and tropolone. Measurements were made using a pulsed beam Fourier transform microwave spectrometer. Rotational and centrifugal distortion constants were determined from the microwave spectra. In the case of the systems that exhibit electric quadrupole coupling interactions, the electric quadrupole coupling strengths were also determined from the analysis of the hyperfine structure in the spectra, yielding additional electronic structure information for the molecules studied. The spectra were also measured for a number of unique, singly substituted isotopologues under natural abundance concentrations. This isotopologue data is crucial in order to obtain key gas phase molecular structure parameters of the molecules and complexes studied. Many theoretical computations, using ab initio and DFT methods, were also performed to obtain optimized electronic structures of the systems studied. These computations aid in the search and assignments of the rotational transitions measured. Comparisons between the theory and the experimental results are described in greater detail in the respective chapters for those systems. The experimental results for the organic systems studied agreed well (within a few percent) with the gas phase optimization computations performed.
16
Willis, Peter G. "DESIGNING MOLECULAR RECOGNITION IN THE CONTEXT OF HYDROGEN BONDING AND MOLECULAR DYNAMICS". UKnowledge, 2001. http://uknowledge.uky.edu/gradschool_diss/279.
Testo completoAbstract (sommario):
The effect of hydrogen bonding on the conformation of organic moleculesunifies two projects in this thesis. In one project, the stability of the intramolecularhydrogen bond in derivatives of 2-guanidinobenzimidazole was studied bydynamic 1H NMR spectrometry. The impact that this intramolecular hydrogenbond had on the bond order of the neutral guanidino group and on the dynamicconformation of these aromatic structures was related to the concept of hydrogenbond-assisted resonance. In another project, an oligomer possessing repetitiveconformation and capable of much inter- and intramolecular hydrogen bondingwas designed and synthesized. The sensitivity of this oligomer to changes inanion concentration, as well as its own propensity to self-aggregate weremeasured.Hydrogen bonds found in many biological oligomers are connected thougha system of conjugated bonds. Guanidinobenzimidazole is a conjugated systemof carbon and nitrogen, connected by an intramolecular hydrogen bond. Severalderivatives of guanidinobenzimidazole were synthesized, and the effect ofseveral simple alkyl for hydrogen substitutions were studied.Guanidinobenzimidazole was used as a model to study what effect theconjugation and the intramolecular hydrogen bond have on each other.The formation of redundant low energy hydrogen bonds is universal inbiological oligomers. In DNA and RNA multiple hydrogen bonds are formed witha typical energy contribution of only 1-2 kcal/mol. Individually, these interactionsdo not give the biological oligomers their conformational stability, but togetherthey are very stable. The urea and amide based oligomers designed in the workand discussed in the thesis should form multiple hydrogen bonds withthemselves and/or with anionic guests. Chiral oligoureas were designed topossess this characteristic of cooperative conformation that so many biologicaloligomers and polymers share.
17
Galamba, Joseph. "Model of the One-Dimensional Molecular Hydrogen Cation". Oberlin College Honors Theses / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=oberlin1337904721.
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18
Kreckel, Holger. "Internal excitations of stored triatomic hydrogen molecular ions". [S.l. : s.n.], 2003. http://deposit.ddb.de/cgi-bin/dokserv?idn=968519563.
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19
Mileeva, Zhanna. "Production and characterisation of molecular hydrogen storage materials". Thesis, University of Salford, 2011. http://usir.salford.ac.uk/26818/.
Testo completoAbstract (sommario):
The subject of the present work is the characterization of potential materials for molecular hydrogen storage with a stress on the use of Small Angle Neutron Scattering (SANS). This research is dedicated mainly to a study of the porous structure of activated carbon. Employment of the contrast matching technique together with SANS led to an understanding of the process of pore filling resulting from an increase of partial pressure of contrast matching liquid (deuterated toluene). The Author has designed a special Al alloy sample cell for the variable vapour pressure SANS experiments. The accessible empty pores fraction at each p/po is calculated using the Porod Invariant. The data obtained matches the results from gravimetric measurements of D-toluene adsorption. The fractal nature of activated carbon is determined via application of the neutron scattering technique. The density of activated carbon - an important characteristic affecting the total hydrogen uptake - is found to be Q-dependent with an average saturation limit of 1.85 g/cc. The effect of the carbon activation process, i.e. the formation of micropores, is illuminated by SANS. Finally, a novel model, implying exponential decay of the pore size distribution with a lower cut-off, is proposed and the minimum pore radius is calculated for each experimental partial pressure of wetting liquid. These results are compared with those derived from the standard Guinier approximation. The proposed model yields the exact value of D-toluene surface tension when the derived pore radii are associated to the corresponding pressures via the Kelvin equation (within the range of its applicability), whereas the Guinier approximation gives an average value of the D-toluene surface tension approximately twice the tabulated value. Thus, it is concluded that the novel model presented in this thesis is an improved approximation to the porous structure of activated carbon. Additionally, a complimentary double-Gaussian pore size distribution model is suggested. It highlights the presence of ultramicro- and microporosity and shows a good agreement with the SANS data for dry activated carbon.
20
Miao, Ling. "Molecular simulations of Pd based hydrogen sensing materials". [Tampa, Fla] : University of South Florida, 2006. http://purl.fcla.edu/usf/dc/et/SFE0001773.
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21
Walker, Martin. "A computational study of hydrogen-bonded molecular crystals". Thesis, University of Edinburgh, 2007. http://hdl.handle.net/1842/1557.
Testo completoAbstract (sommario):
The current climate in research has focused on the rational design of new materials with desirable characteristics. The demand for their full characterisation has in turn placed a new importance on structural chemistry, and important developments have taken place as a consequence. For instance an important probe to understand the interactions between molecules is to use variable pressure, and this has been exploited experimentally through the design and implementation of the diamond anvil compression cell (DAC). Using a DAC to study molecular materials at high pressure can result in problems, however: in X-ray diffraction the physical presence of the cell restricts access to reciprocal space, resulting in experimental structures of lower precision and often missing hydrogen atom location data. Traditionally the solution has been sought in neutron diffraction where hydrogen (deuterium) atoms scatter more intensely and so contribute more to the scattering pattern. This introduces another set of problems, however, in that the assumption is made that the isotope substitution does not alter the overall structure. In addition expense and time delays are incurred through this protracted experimental route. This thesis reports the development of a computational technique which can be used to reliably locate hydrogen atoms without the need for neutron diffraction data. The project reports rigorous testing on cases of varying difficulty, from the simple to the more complex. The test cases selected were also of industrial and environmental importance, so determining their complete structures under high pressure conditions was in itself a desirable outcome. Computationally completed structures were then compared to neutron diffraction results or used as the model to be refined against the neutron diffraction pattern.
22
Trevisan, Cynthia Sandra. "Near threshold electron impact dissociation of molecular hydrogen". Thesis, University College London (University of London), 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397950.
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23
Murata, Michihisa. "Organic synthesis of endohedral fullerenes encapsulating molecular hydrogen". 京都大学 (Kyoto University), 2006. http://hdl.handle.net/2433/64942.
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24
Wang, Liang-Guo. "Studies of Rydberg atomic xenon and molecular hydrogen /". The Ohio State University, 1986. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487323583622812.
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25
CECCONI, BIANCA. "Artificial Photosynthesis: Molecular Approaches for Photocatalytic Hydrogen Production". Doctoral thesis, Università degli Studi di Milano-Bicocca, 2016. http://hdl.handle.net/10281/100472.
Testo completoAbstract (sommario):
The humankind today needs to face an epochal transition from a fossil fuel to a renewable source-based economy. Renewable sources are our chance to build a clean world with unlimited and widespread energy. Nowadays renewable energies could be properly harvested to produce electricity, while the development of a future clean fuel is less advanced. Since our energetic consumption is made essentially of fuels we need to build devices to transform renewable energy, such as solar radiation, into chemical energy of bonds. A promising future fuel is hydrogen since its carbon footprint is zero and it can be obtained from an abundant source such as water. Nature, through the photosynthesis, could inspire us to build our feed in the form of fuels. In this research project DSSC (dye-sensitized solar cells) have been modified to produce chemical energy instead of electricity. Attention has been focused on hydrogen production semi-reaction, thus the use of a sacrificial electron donor has been adopted. Such system is composed of TiO2 nanoparticles covered by a reduction catalyst and a metal-free organic sensitizer to harvest the visible spectrum of solar radiation.
The aim of this research has been the development of molecular approaches to provide efficient light harvesting systems and reduction catalysts. Molecular design allowed a fine tuning of materials properties and a deep understanding of structure/performances relationships.
The first part of the project has focused on designing push-pull structures to harvest visible light portion of solar spectrum. Fine molecular tuning of metal-free dyes afforded enhanced performances depending on the kind of modification. We modified a known phenothiazine dye in the donor, spacer and acceptor units in order to derive structure/performances relationships. Enhanced light harvesting properties and photo-stability have been afforded through π-spacer modification with various mono- and polycyclic simple and fused thiophene derivatives, while decoration of the donor core with glycolic or sugar chains gave better hydrophilicity and surface wettability. Lastly hydroxamic acids have been introduced as alternative anchoring groups to give stronger ester bonds on TiO2 surface and prevent hydrolysis in aqueous media.
The second part of the research has concerned the study of cobaloximes as alternative noble metal free reduction catalysts. Starting from a mini-library of cobaloximes bearing various equatorial bridges, axial ligands, and starting oxidation numbers, molecular structure/efficiency studies have been done, while UV/Vis spectroscopy has been used to investigate the nature of the eventual Co(I) species transiently formed. For cobaloximes a Co(I) species is hypothesized but not confirmed in photocatalytic experiments and optimization of efficiency and stability of new catalysts need a deep understanding of the catalytic cycle in order to intervene in the critical intermediates.
26
Smith, Susan Lavinia. "Chiral selection in hydrogen atom transfer reactions". Thesis, Royal Holloway, University of London, 1986. http://repository.royalholloway.ac.uk/items/f4ae87be-b444-42db-828c-b12089967742/1/.
Testo completoAbstract (sommario):
This thesis describes an attempt to demonstrate enantioselectivity in free-radical hydrogen transfer reactions and was designed to test a novel extension of the Hammond Postulate. It is proposed that the rates of two related reactions which are thermodynamically identical but kinetically distinct should differ most when the processes are thermoneutral since the transition state is remote from both reactant and product. Bysynthesis and subsequent oxidation of suitable chiral hydroxamic acids a series of persistent chiral acyl nitroxide radicals ArCON(R)O has been made. By reacting such a series of nitroxide radicals with suitable chiral benzylic alcohols ArCHOHR it was hoped that by varying the nature of, the bond strength of the alcohol would come within the compass of the 0-H bond strengths of ArCON(R)OH. It was predicted that such an alcohol should show a maximum in a plot of enantioselectivity vs. 0-H bond strength. The target molecules for this work were optically active N-alkylbenzohydroxamic acids in which the akyl substituent was both chiral and tertiary. Substitution of electron withdrawing and releasing groups, ranging from into the aroyl group of the hydroxamic acid and subsequent oxidation of the latter allowed the synthesis of radicals with varying bond strengths. UV-visible, e.s.r. and CD spectra were determined for these radicals. 0-H bond strengths of the hydroxamic acids were estimated using an e.s.r. technique which determined the equilibrium position for hydrogen atom transfer between the hydroxamic acid and a standard di-t-alkyl nitroxide radical which forms a bond of known strength to hydrogen. These estimated values range from 76 to 79 kcal mol. Enantoselectiviy was searched for in two ways. The first employed UV spectroscopy to determine the second order rate constants for the four possible reaction pairs of chiral nitroxide enantiomers with benzylic alcohol enantiomers. The second method involved reaction of racemic alcohol with chiral radical and subsequent examination of enantiomeric excess by high performance liquid chromatography (HPLC); two approaches to this are described. In the majority of cases studied, however, enantioselectivity was immeasurably small. In the case of 2-methyl-l-phenylpropan-l-ol, a small enantiomeric excess was observed but further work is needed to substantiate these results. Small chiral discriminations were observed in the oxidation reactions of only one alcohol. These results were insufficient to investigate the extension of the Hammond Postulate as originally planned.
27
Axelsson, Rikard. "Cyanobacterial Hydrogen Metabolism : Transcriptional Regulation of the Hydrogenases in Filamentous Strains". Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3590.
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Hunt, Maria, University of Western Sydney, of Science Technology and Environment College e School of Engineering and Industrial Design. "Molecules in southern molecular clouds: a millimetre-wave study of dense cores". THESIS_CSTE_EID_Hunt_M.xml, 2001. http://handle.uws.edu.au:8081/1959.7/116.
Testo completoAbstract (sommario):
This thesis presents an observational study of molecular abundances in the dense cores of 27 prominent molecular clouds in the southern galactic plane.The molecular abundances and physical conditions in dense condensations have been derived from millimetre-wavelength observations of molecular rotational transitions.The study has produced a comprehensive data set of transition intensities and abundances for 10 different molecules in bright southern molecular clouds, and the general characteristics of emissions from these molecules such as optical depth, excitation and relative abundances are discussed. A comparison of different methods of calculating molecular hydrogen column density from observations of carbon monoxide emission is included.Both the analysis and the data collected provide an excellent starting point for further observational and theoretical studies of molecular clouds in the southern Milky Way utilising new instruments such as the millimeter-wave upgrade to the Australia Telescope Compact Array and the Attacama Large Millimetre Array (ALMA).Doctor of Philosophy (PhD)
29
Nalbantoglu, Tugrul. "Towards a hydrogen bond mediated directional walker and light driven molecular shuttles". Thesis, University of Manchester, 2017. https://www.research.manchester.ac.uk/portal/en/theses/towards-a-hydrogen-bond-mediated-directional-walker-and-light-driven-molecular-shuttles(fb210b38-4079-4376-942e-b907a8f82d3f).html.
Testo completoAbstract (sommario):
This thesis reports the efforts towards the design and synthesis of a small molecule walker that would potentially move along the track directionally by exploiting the secondary interactions between the track and the walker. This thesis also reports the synthesis and operation of a light driven molecular shuttle featuring a novel acylpyridyl hydrazone station. Chapter One describes the biological walkers which are the source of inspiration towards the synthetic walkers, characteristics of a walker, previously described small molecule walkers and recent progress on the synthesis of molecular shuttles that operate under variety of different stimuli. Chapter Two describes the design and synthetic efforts towards a molecular walker that has the potential to operate directionally along the track by exploiting secondary interactions between the walker and the track namely the hydrogen bonding interactions introduced by subtle incorporation of excellent hydrogen bond donor/acceptor squaramides. This chapter briefly mentions the hydrogen bonding capabilities of squaramides on which the directional operation relies. Optimization of critical reactions and attempted strategies for the assembly of the whole machine is described as well. Chapter Three describes the synthesis and operation of 1- and 2- station [2]-rotaxanes that operate under light irradiation. 2- station [2]-rotaxane that function as a light driven molecular shuttle presents remarkable positional fidelity with high efficacy. The bistable acyl pyridyl station is incorporated as a photo active station upon which light irradiation alters the binding affinities towards the macrocycle. Series of rotaxanes featuring different amide based stations were synthesized to determine the best non-photo active station.
30
Mccartney, Michael Stephan Karl. "Fluorescent molecular hydrogen in the reflection nebula NGC 2023". Thesis, University of Edinburgh, 1998. http://hdl.handle.net/1842/28563.
Testo completoAbstract (sommario):
Photodissociation regions (PDRs) are described in the context of the interstellar medium and star-forming regions. Observations of PDRs and molecular hydrogen are reviewed and the reflection nebula NGC 2023 is discussed in detail. NGC 2023 is a bright and well-studied reflection nebula at a distance of 450 parsecs in the Orion region. Illuminated primarily by a B-type star, it offers an ideal opportunity to study UV-excited molecular hydrogen. The theory of the hydrogen molecule is described: the energy states and their relationship with the quantum numbers which represent the vibratinal and rotational states of the molecule, the radiative processes which determine the optical and infrared emission spectrum of H2, the effect collisions have on the excited states of the molecule and the processes which govern the formation and destruction of H2. Particular attention is given to the process of formation on the surface of dust grains and the resulting energy states of the ejected H2 molecule. Infrared and optical far-red observations of fluorescent H2 line emission from NGC 2023 are presented. The resulting datasets contain flux measurements of over ninety lines. These are combined with published data to produce column densities for 81 energy states of the H2 molecule, the most extensive dataset yet compiled for a PDR. The process of observing in the infrared red optical wavelength regimes are outlined. The emission lines of H2 are intrinsically very faint and thus measurements require careful data reduction to minimise sources of noise wherever possible. The data reduction steps which were applied to observations are described in detail. An optical extinction of Av = 5.7 ? 0.5 to the H2 emission region and ortho/para abundance ratio of 2.0 ± 0.2 are derived from flux ratios of emission lines and by minimising the scatter on a diagram which plots the logarithm of the column density against the energy level of each state.
31
Carr, James M. "Low energy positron scattering from the hydrogen molecular ion". Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363939.
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Conroy-Lewis, F. M. "Synthesis and reactivity of molecular hydrogen complexes of ruthenium". Thesis, University of Salford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381680.
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33
Boissonnet, Michel-Franck. "Hydrogen-bonding ferrocene derivatives for molecular recognition and organocatalysis". Thesis, University of Birmingham, 2015. http://etheses.bham.ac.uk//id/eprint/6121/.
Testo completoAbstract (sommario):
The aim of this project was to prepare novel ferrocene-based Hydrogen-bonding receptors and to study them in the electrochemical sensing of neutral compounds and in organocatalysed transformations. The synthesis and characterisation of 2-ferrocenyl oxazoles via gold chemistry to access a new H-bonding motif for electrochemical sensing was successfully achieved. However the targeted structure bearing a secondary amine at the 4-position of the oxazole, was found to be highly unstable and unsuitable for sensing applications. Further studies on ferrocene-based H-bonding systems towards their application in asymmetric organocatalysis were also carried out. Different chiral and achiral ferrocene-(thio)ureas were prepared and tested in four organic reactions. Their performances in the Henry and in the Morita-Baylis-Hillman reactions gave acceptable yields but did not show significant enantioselectivities. A bi-functional ferrocene-thiourea was found to be effective in the enamine co-catalysed aldol reaction of acetone and \(trans-β\)-nitrostyrene, and in the double Michael cycloaddition of ω-nitropentenoate methyl and \(trans-β\)-nitrostyrene, leading to a tetrasubstituted cyclopentane. The H-bonding properties of the ferrocene-(thio)ureas with carboxylates was also studied by \(^1\)H NMR spectroscopy. Finally, approaches towards the preparation of ferrocene-based boronate-ureas were investigated.
34
Islam, F. "The formation of molecular hydrogen in the interstellar medium". Thesis, University College London (University of London), 2010. http://discovery.ucl.ac.uk/19475/.
Testo completoAbstract (sommario):
H2 is the most abundant molecule in the interstellar medium and forms on the surface of interstellar dust grains. Laboratory studies have been conducted of HD formation on a dust grain analogue, which is a highly-oriented pyrolytic graphite surface held at 15 K, under ultra-high vacuum. The molecules desorb from the surface in a distribution of ro-vibrational states, which are probed using Resonance Enhanced Multi-Photon Ionization Spectroscopy. HD in a particular ro-vibrational state is ionized using laser photons detected by a time-of-flight mass spectrometer. The HD+ ion yields are then data processed to obtain the relative rotational populations of HD formed within one vibrational level and an average rotational temperature can be found. In this thesis, HD formed in vibrational states v = 3 – 7 have been studied. This carries on from previous studies of HD and H2 in the v = 1 and 2 states. Within each vibrational level, the most populated rotational state was found to be J = 1 or 2. The most populated vibrational state was found to be v = 4. The HD experimental results were extrapolated to give the relative ro-vibrational population distribution of nascent H2, which provides a new model for the formation pumping of H2. This new formation pumping model has been implemented into a radiative transfer code, written by Casu and Cecchi-Pestellini, which takes into account formation, radiative and collisional pumping mechanisms to calculate the total population distribution of H2 in an interstellar cloud and to generate H2 spectra. The sensitivity of the H2 spectra to the physical conditions of interstellar dark clouds, such as cloud density and temperature, has been investigated. H2 spectra generated using the new experimentally-derived formation pumping model has also been compared to H2 spectra generated using other established, theoretically-derived formation pumping models.
35
Garfitt, Jason Michael. "Hydrogen bonding and covalent coupling in adsorbed molecular monolayers". Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/27728/.
Testo completoAbstract (sommario):
This thesis examines multiple different molecular networks adsorbed on several different substrates, namely, highly oriented pyrolytic graphite (HOPG), Au(111) and graphene. STM investigations into hydrogen-bonded structures formed by closely related tetracarboxylic acid molecules were performed. The molecule pterphenyl-3,5,3,5-tetracarboxylic acid (TPTC), which is known to form random tiling networks, was observed on a graphene on copper substrate. The network formed from deposition of TPTC from nonanoic acid was examined statistically. Aqueous solutions of TPTC were also examined on HOPG where a new structure, distinct from the random tiling, was observed. Aqueous solutions of related molecules biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC) and quaterphenyl-3,3',5,5'-tetracarboxylic acid (QPTC), were also studied on HOPG. QPTC formed a similar structure to the aqueous solutions of TPTC, but BPTC formed two different phases, one of which was a kagome network. Addition of nonanoic acid to a dried network of TPTC deposited from aqueous solution resulted in solvent induced recrystallisation into a random tiling network comparable to that observed on graphene on copper, which was statistically analysed. Studies investigating the potential for covalent bonded molecular networks identified two distinct phases of the molecule 1,3,5-Tri(4-bromophenyl-benzene (TBPB)) adsorbed on Au(111). Concentration variation indicates an island based growth mechanism for these domains from solution. Dimerisation of TBPB was achieved by deposition onto heated substrates and a discussion of possible reasons for the reaction termination at dimers is provided. Attempts to repeat the TBPB experiments on graphene on copper failed due to excessive corrosion. Variations using larger molecules failed due to lack of solubility. Preliminary experiments on 10,10'-dibromo-9,9'-bianthryl (DBrBA) showed promise but were irreproducible, however micron scale dendritic structures were observed suggesting poor compatibility with the solvent. Finally, a discussion of the development of a nickel catalysis based graphene fabrication method is given and the limits of what is achievable with this method are discussed. The results from this thesis highlight the importance of solvent selection for the future understanding of molecular network fabrication. We also demonstrated the feasibility of covalently bonded networks prepared in ambient conditions.
36
Schiavi, Andrea. "Investigation of vibrating-hydrogen based ultrashort molecular phase modulator". Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:94c3e831-9045-4d32-acc3-5ef7c6bca135.
Testo completoAbstract (sommario):
This thesis investigates the coherent phase modulation of ultrashort pulses using vibrating hydrogen as a molecular medium. Self-phase modulation in a gas-filled hollow core capillary allows the generation of highpower few-cycle pulses in the NIR. Such pulses can be used to drive high harmonic generation (HHG) to deliver attosecond duration pulses in the extreme ultraviolet and soft X-ray spectral region. While reaching unrivalled pulse durations (down to 67 as), these sources have characteristically low efficiencies. The pump-probe spectroscopy community would greatly benefit from brighter short wavelength sources with sub-5 fs duration. In this work I apply Amplified RamaN Impulsive Excitation for Molecular Phase Modulation (ARNIEMPM), a multiple pulse scheme, to coherently prepare vibrating hydrogen molecules and exploit them for the phase modulation of ultrashort pulses. The preparation of the molecular motion is performed via impulsive stimulated Raman scattering and transient stimulated Raman scattering. The generated in-phase motion of molecules creates an oscillating optical polarizability in the medium which can be exploited by a probe pulse propagating through it, acting as a 125THz frequency phase modulator, the fastest among molecular media. This technique has the potential to provide bright, isolated subfemtosecond duration ultra-violet (UV) pulses via spectral broadening of broadband pulses. I experimentally investigate the preparation of the molecular motion against multiple experimental parameters. I then demonstrate the molecular phase modulation of ultrashort broadband probes in the near-infrared (NIR) and UV via a degenerate interferometric scheme. I used a waveguide to increase the interaction length of the process and reduce the energy requirements for the medium preparation. This allowed the use of a single laser system to generate all the required pulses, which are largely diverse in terms of wavelength, duration and power. Additionally, I present a novel technique named Attosecond Resolved Interferometric Electric-field Sampling (ARIES), which is capable of directly measuring the waveform of arbitrary pulses with attosecond resolution. This technique is based on high-harmonic generation (HHG) acting as a temporal gate for an applied secondary field, and tracking its electric field amplitude as a shift in the HHG cut-off frequency. I present experimental demonstration of a pulse waveform measurement by accurately retrieving a know inserted variation in dispersion and carrier-envelope-phase. A theoretical calculation of the technique applicability over a wide spectral range is also presented.
37
Garrett, Benjamin R. "MoO(S2)2L Based Molecular Electrocatalysts for Hydrogen Production". The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1490964132615695.
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38
Irving, Benjamin James. "The formation of molecular hydrogen in the interstellar medium". Thesis, University of Sheffield, 2012. http://etheses.whiterose.ac.uk/3104/.
Testo completo
39
Clark, Jessica Higgins Deseret. "The ortho to para transition in muonic molecular hydrogen". W&M ScholarWorks, 2002. https://scholarworks.wm.edu/etd/1539623408.
Testo completoAbstract (sommario):
A precise measurement of the induced weak pseudoscalar coupling constant of the proton, gp, is of interest as a basic test of chiral symmetry breaking. This is the least well-known weak form factor of the nucleon. Muon capture experiments [Jonkmans et al., Phys. Rev. Lett. 77(1996)4512] have been used to measure gp. However, the interpretation of these muon capture experiments requires knowledge of the relative population of the muonic atomic and molecular states for muons in liquid hydrogen. In particular, lambda op, the transition rate between the ortho- and para-molecular states of the pmup molecule, needs to be known with precision. The only previous measurement [Bardin et al., Phys. Lett. 104B(1981)320.] of lambda op yielded a value of (4.1 +/- 1.4) x 10 4 s-1, significantly different than the theoretical prediction [Bakalov et al., Nucl. Phys. A384(1982)302.] of 7.1 x 104 s-1. A measurement of lambdaop using the time distribution of neutrons from muon capture was performed at TRIUMF during June--July 1999 and November 1999. The lambdaop measured in this experiment is (13.8 +/- 1.6) x 104 s-1 . The uncertainty is dominated by the statistical precision of the fit and the background due to the deuterium contamination in the protium target. The implications of this result for gp are discussed.
40
Vanne, Yulian V. "Ionization of molecular hydrogen in ultrashort intense laser pulses". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2010. http://dx.doi.org/10.18452/16107.
Testo completoAbstract (sommario):
Ein neuer numerischer ab initio Ansatz wurde entwickelt und zur Lösung der zeitabhängigen Schrödingergleichung für zweiatomig Moleküle mit zwei Elektronen (z.B. molekularer Wasserstoff), welche einem intensiven kurzen Laserpuls ausgesetzt sind, angewandt. Die Methode basiert auf der Näherung fester Kernabstände und der nicht-relativistischen Dipolnäherung und beabsichtigt die genaue Beschreibung der beiden korrelierten Elektronen in voller Dimensionalität. Die Methode ist anwendbar für eine große Bandbreite von Laserpulsparamtern und ist in der Lage, Einfachionisationsprozesse sowohl mit wenigen als auch mit vielen Photonen zu beschreiben, sogar im nicht-störungstheoretischen Bereich. Ein entscheidender Vorteil der Methode ist ihre Fähigkeit, die Reaktion von Molekülen mit beliebiger Orientierung der molekularen Achse im Bezug auf das linear polarisierte Laserfeld in starken Feldern zu beschreiben. Dementsprechend berichtet diese Arbeit von der ersten erfolgreichen orientierungsabhängigen Analyse der Multiphotonenionisation von H2, welche mit Hilfe einer numerischen Behandlung in voller Dimensionalität durchgeführt wurde. Neben der Erforschung des Bereichs weniger Photonen wurde eine ausführliche numerische Untersuchung der Ionisation durch ultrakurze frequenzverdoppelte Titan:Saphir-Laserpulse (400 nm) präsentiert. Mit Hilfe einer Serie von Rechnungen für verschiedene Kernabstände wurden die totalen Ionisationsausbeuten für H2 und D2 in ihren Vibrationsgrundzuständen sowohl für parallele als auch für senkrechte Ausrichtung erhalten. Eine weitere Serie von Rechnungen für 800nm Laserpulse wurde benutzt, um ein weitverbreitetes einfaches Interferenzmodel zu falsifizieren. Neben der Diskussion der numerischen ab initio Methode werden in dieser Arbeit verschiedene Aspekte im Bezug auf die Anwendung der Starkfeldnäherung für die Erforschung der Reaktion eines atomaren oder molekularen Systems auf ein intensives Laserfeld betrachtet.A novel ab initio numerical approach is developed and applied that solves the time-dependent Schrödinger equation describing two-electron diatomic molecules (e.g. molecular hydrogen) exposed to an intense ultrashort laser pulse. The method is based on the fixed-nuclei and the non-relativistic dipole approximations and aims to accurately describe both correlated electrons in full dimensionality. The method is applicable for a wide range of the laser pulse parameters and is able to describe both few-photon and many-photon single ionization processes, also in a non-perturbative regime. A key advantage of the method is its ability to treat the strong-field response of the molecules with arbitrary orientation of the molecular axis with respect to the linear-polarized laser field. Thus, this work reports on the first successful orientation-dependent analysis of the multiphoton ionization of H2 performed by means of a full-dimensional numerical treatment. Besides the investigation of few-photon regime, an extensive numerical study of the ionization by ultrashort frequency-doubled Ti:sapphire laser pulses (400 nm) is presented. Performing a series of calculations for different internuclear separations, the total ionization yields of H2 and D2 in their ground vibrational states are obtained for both parallel and perpendicular orientations. A series of calculations for 800nm laser pulses are used to test a popular simple interference model. Besides the discussion of the ab initio numerical method, this work considers different aspects related to the application of the strong-field approximation (SFA) for investigation of a strong-field response of an atomic and molecular system. Thus, a deep analysis of the gauge problem of SFA is performed and the quasistatic limit of the velocity-gauge SFA ionization rates is derived. The applications of the length gauge SFA are examined and a recently proposed generalized Keldysh theory is criticized.
41
Woodman, George Henry. "Precise laser spectroscopy of atomic hydrogen". Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316894.
Testo completo
42
Frost, Mungo David. "Simple molecular systems under pressure". Thesis, University of Edinburgh, 2016. http://hdl.handle.net/1842/25788.
Testo completoAbstract (sommario):
Under pressure small molecular systems exhibit a remarkable degree of polymorphism and unexpected behaviours. Two previously unexplored binary systems, those of niobium{hydrogen and water{oxygen, are explored and their phase diagrams mapped. In the case of water and oxygen, two hitherto unknown clathrate structures are found. A wide study of dense nitrogen at and above room temperature is also reported. The phase diagram of nitrogen is exceptionally complex for a single element and exhibits considerable metastability of phases. Current theoretical understanding of nitrogen has many disagreements with experimental observation. High quality structural data on various previously known phases are reported as well as a novel molecular phase. This new phase, λ-N2, has been studied using Raman spectroscopy and powder x-ray diffrraction. Combining experimental and theoretical results gives a layered structure with intermolecular interactions playing an important role and an apparently weakened intramolecular bond. The dissociation of the nitrogen triple bond is also examined. The amorphous η state is studied via Raman spectroscopy and optical and IR absorbance methods before laser heating at 255 GPa. After heating the sample becomes very much more transparent and shows signs that the nature of the bandgap has changed suggesting that the nitrogen may be in a crystalline atomic phase hitherto unknown. An effort to take nitrogen to considerably higher pressures is reported. A two stage diamond anvil cell was developed using focused ion beam micromachining. Although ultimately unsuccessful, to the author's knowledge this was the first ever attempt at designing such a system to be compatible with small molecular samples and Raman spectroscopy and various conclusions are as to how best to proceed with such developments.
43
Pohl, Vincent [Verfasser]. "Electronic Motion in Molecular Systems: From the Hydrogen Molecular Ion to Nanostructures / Vincent Pohl". Berlin : Freie Universität Berlin, 2017. http://d-nb.info/1144270278/34.
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44
Shaw, John A. "Photoabsorption spectra of hydrogen in magnetic fields". W&M ScholarWorks, 1993. https://scholarworks.wm.edu/etd/1539623837.
Testo completoAbstract (sommario):
Measurements of the absorption spectrum of atomic H in strong magnetic fields have been analyzed. The measurements, performed by the Bielefeld, Germany experimental group, investigated the photoabsorption to levels near the ionization threshold in magnetic fields ranging from 2.7 to 6 Tesla. Taking advantage of a classical scaling law, the photon energy and the magnetic field strength were varied simultaneously in the experiment and the absorption rate vs. B{dollar}\sp{lcub}-1/3{rcub}{dollar} at fixed scaled-energy, {dollar}\varepsilon{dollar} = E/(B/B{dollar}\sb{lcub}\rm o{rcub})\sp{lcub}2/3{rcub}{dollar} was measured. The absorption rate was observed to exhibit sinusoidal fluctuations which we correlate with closed classical orbit of the electron. A Fourier transformation of this signal yields peaks which we interpret as "recurrence strengths" which depend upon the classical action of the closed orbit. Closed-orbit theory gives formulas for these recurrence strengths. as the scaled energy is increased, observed recurrences proliferate, consistent with the change from orderly to chaotic motion of the electron. Bifurcation theory provides organizing principles for understanding this proliferation and for interpreting the data. New "exotic" orbits suddenly appear "out of nowhere" through saddle-node bifurcations. The "main sequence" of orbits is produced from an orbit parallel to B through a sequence of pitchfork and period-doubling bifurcations. Other recurrences are created by period-tripling and higher-order bifurcations of existing orbits. These bifurcations can have generic structure, or sometimes the structures are modified by symmetries of the system. Focusing effects associated with these bifurcations cause some recurrences to be particularly strong.
45
Steel, Stephen Chris. "Magnetic resonance studies of atomic hydrogen confined by solid molecular hydrogen between 6.4 and 8.2 k". Thesis, University of British Columbia, 1985. http://hdl.handle.net/2429/25046.
Testo completoAbstract (sommario):
Magnetic hyperfine resonance in zero magnetic field has been used to study atomic hydrogen gas confined by walls coated with solid molecular hydrogen at temperatures between 6.4 and 8.2 K. The temperature range of the experiments was limited by the nature of the atom source used to produce atoms.
Measurements of the frequency shift at low atom densities (nH ≃ 8 x l0⁺¹⁶ m⁻³) have yielded a binding energy for H on H₂ of 34.04 ± 0.26 K, and a surface frequency shift of -1.16 ± 0.05 MHz. These results are in excellent agreement with those obtained by Crampton et al between 3.2 and 4.5 K. The pressure shift due to the vapour pressure of the solid H₂ was found to be -1.78 ± 0.01 x 10⁻²⁴ Hz m³.
The low atom density transverse relaxation rate measurements are difficult to interpret. There seems to be a relaxation mechanism on the H₂ surface that gives a contribution beyond that associated with the dispersion of frequency shifts an atoms see from normal adsorption. The data does show that the sticking coefficent of H on H₂ is greater than 0.04.
Measurements at higher atom densities (nH ≤ 9 x 10⁺¹⁸ m⁻³) gave values for the surface recombination cross length which increased from 0.5 A at 6.4 K to 1.1 A at 8.2 K. The bulk spin exchange cross section was found to agree quite well with the calculations of Berlinsky and Shizgal.Science, Faculty of
Physics and Astronomy, Department of
Graduate
46
Banu, Ana Maria. "Molecular simulation studies of metal organic frameworks focusing on hydrogen purification". Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/8983.
Testo completoAbstract (sommario):
The process of purifying hydrogen gas using pressure swing adsorption columns heavily relies on highly efficient adsorbents. Such materials must be able to selectively adsorb a large amount of impurities, and must also be regenerated with ease. The work presented in this thesis focuses on a novel class of porous solids, metal-organic frameworks (MOFs), and their potential for use as adsorbents in hydrogen purification processes. MOFs are tuneable structures, a property that can be exploited in order to achieve the desired characteristics that are beneficial for a specific application. The design or selection of MOFs for any separation process however, relies on a thorough understanding of the relationship between a framework’s characteristics and its adsorption and selective properties. In order to identify favourable MOF characteristics for the separation of hydrogen from typical impurities a systematic molecular simulation study is performed on a large group of MOFs. Features such as the presence of short linkers, amine groups and additional aromatic rings, and a high density of linker groups are found to increase the adsorbate - framework interaction strength, and reduce the free volume available inside the pores. Both of these effects are shown to enhance MOF selectivity for impurities. Two promising materials, exhibiting desirable features, Mn MIL-53 and MIL-47, are studied further through a variety of approaches. A combination of experimental work and molecular simulations are employed in order to assess the level of flexibility in Mn MIL-53 on uptake of CO2 and CH4. An investigation of the experimental and simulation adsorption and characterization data indicates that the framework undergoes structural changes, in order to accommodate CO2 molecules, but not CH4. The form of the framework during CO2 uptake is also shown to be strongly influenced by temperature. In the case of MIL-47, adsorption isotherms simulated for a wide range of gases overpredict experimental adsorption data, leading to an in-depth investigation of non-porous effects, force field suitability, and framework rigidity. Ab initio molecular dynamics studies of MIL-47 indicate that the benzene dicarboxylate linkers rotate about their symmetry axis to reach more energetically favourable configurations, an effect responsible for the discrepancies between simulated and experimental isotherms. The effect of MOF flexibility on adsorption is further highlighted in a study of Sc2BDC3, a material able to undergo structural changes in order to accommodate a variety of adsorbates. Molecular simulations show that structural changes in the framework are responsible for the creation of additional CO2 adsorption sites as pressure is increased, whereas methanol adsorption sites occupied at extreme pressure are stabilized by the formation of hydrogen bonds. Finally, the exceptionally robust UiO-66(Zr) and UiO-67(Zr) families of MOFs are analysed using a multi-scale simulation study combining molecular level and process-scale computational work, seeking to compare the materials to commercial adsorbents, and assess whether they are suitable for H2 purification through pressure swing adsorption (PSA). Of the four MOFs studied, UiO-66(Zr)-Br is the most promising, as it significantly outperforms commercial zeolites and activated carbons in H2 purification from steam methane reformer offgas.
47
Farebrother, Adam John. "Formation of diatomic molecules at surfaces". Thesis, University College London (University of London), 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.272445.
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Perry, James Samuel Anthony. "State selective reactions of cosmic dust analogues at cryogenic temperatures". Thesis, University College London (University of London), 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.252206.
Testo completo
49
Gokcen, Taner. "Molecular engineering of trigonal octupolar materials based on 2,4,6-diarylamino-1,3,5-triazines". Link to electronic thesis, 2005. http://www.wpi.edu/Pubs/ETD/Available/etd-082405-155024/.
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Diaconu, Cristian V. "Hybrid density functional studies of hydrogen storage related molecular systems /". View online version; access limited to Brown University users, 2005. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:3174594.
Testo completo