Добірка наукової літератури з теми "Pnicogen bond"

The pnicogen bonding interactions of PCl3and π-electron systems (acetylene, ethylene, benzene) were calculated by using MP2/aug-cc-pVDZ method and the effect of hydrogen bond on pnicogen bond systems were investigated. It has been indicated that the hydrogen bonding and the pnicogen bonding interactions have influence on each other and the positively cooperative effect has been detected. The interaction energies of pnicogen bonded supramolecular system were also calculated by using DFT method (M06-2X) and some simple comparisons with those by using MP2 method were made. It has been disclosed from natural bond orbitals (NBO) analysis that more the amount of charge transfer of pnicogen bonding interaction, the greater the stability of the corresponding complex. Through AIM topological analysis, it has been revealed that the electron density of pnicogen bond BCP point is positively correlated with the stability of trimeric complex. Electron localization function (ELF) was also adopted to analyze the nature of pnicogen bonding interactions. Furthermore, density difference function (DDF) method was adopted to analyze the variation of electron density of pnicogen bond system because of hydrogen bond.

A comparative study of the cooperative effects of hydrogen and pnicogen bonding on open-chain clusters of (PH2CN)n=2–7 and (HCN)n=2–7 is performed at the MP2/6-311++G(d,p) level of theory. These effects are studied in terms of geometric and energetic properties, electron density analysis, and 15N chemical shielding parameters of the clusters at the MP2/6-311++G** level. The intermolecular distances observed in the (HCN)n clusters exhibit quite larger bond contractions than those found in the (PH2CN)n clusters. Our results strongly suggest that cooperative effects induced by pnicogen and hydrogen bonds are significant in both linear PH2CN and HCN clusters, respectively. They also provide some evidence that these effects seem to reach a limit for a relatively small number of monomers. The n-dependent variation in the 15N chemical shielding tensor should serve as a useful signature of cooperativity effects in the PH2CN and HCN clusters.

A theoretical study of the substituent and solvent effects on the reaction of phosphines with CO2 has been carried out by means of Møller-Plesset (MP2) computational level calculations and continuum polarizable method (PCM) solvent models. Three stationary points along the reaction coordinate have been characterized, a pre-transition state (TS) assembly in which a pnicogen bond or tetrel bond is established between the phosphine and the CO2 molecule, followed by a transition state, and leading finally to the adduct in which the P–C bond has been formed. The solvent effects on the stability and geometry of the stationary points are different. Thus, the pnicogen bonded complexes are destabilized as the dielectric constant of the solvent increases while the opposite happens within the adducts with the P–C bond and the TSs trend. A combination of the substituents and solvents can be used to control the most stable minimum.

The variety of interactions have been analyzed in numerous studies. They are often compared with the hydrogen bond that is crucial in numerous chemical and biological processes. One can mention such interactions as the halogen bond, pnicogen bond, and others that may be classified as σ-hole bonds. However, not only σ-holes may act as Lewis acid centers. Numerous species are characterized by the occurrence of π-holes, which also may play a role of the electron acceptor. The situation is complicated since numerous interactions, such as the pnicogen bond or the chalcogen bond, for example, may be classified as a σ-hole bond or π-hole bond; it ultimately depends on the configuration at the Lewis acid centre. The disadvantage of classifications of interactions is also connected with their names, derived from the names of groups such as halogen and tetrel bonds or from single elements such as hydrogen and carbon bonds. The chaos is aggravated by the properties of elements. For example, a hydrogen atom can act as the Lewis acid or as the Lewis base site if it is positively or negatively charged, respectively. Hence names of the corresponding interactions occur in literature, namely hydrogen bonds and hydride bonds. There are other numerous disadvantages connected with classifications and names of interactions; these are discussed in this study. Several studies show that the majority of interactions are ruled by the same mechanisms related to the electron charge shifts, and that the occurrence of numerous interactions leads to specific changes in geometries of interacting species. These changes follow the rules of the valence-shell electron-pair repulsion model (VSEPR). That is why the simple classification of interactions based on VSEPR is proposed here. This classification is still open since numerous processes and interactions not discussed in this study may be included within it.

A computational study of the intramolecular pnicogen bond in 1,8-bis-substituted naphthalene derivatives (ZXH and ZX₂ with Z = P, As and X = H, F, Cl, and Br), structurally related to proton sponges, has been carried out.

The thesis entitled “High-Resolution Charge Density Studies on Electronic Nature of Weak Interactions and Correlation of Molecular Conformation with Packing in Solid State” consist of five chapters. Chapter 1 is a brief introduction to the methodologies and techniques utilized in modelling electron densities and the topics relevant to the work. The subsequent four chapters are divided into two parts-Part A and Part B. Part A has two chapters that discusses the electronic nature of unexplored weak intermolecular interactionspnicogen bonding in nitrogen atom and hydrophobic interactions between methyl groups in molecular crystals. Part B also contains two chapters that investigates the symbiotic relation of molecular conformation and packing in solid state in two unique cases-hybridized induced polymorphism observed in sulfa drug acetazolamide and unusual asymmetry observed in overcrowded Octachloronaphthalene molecule Part A: Electronic Nature of Weak Interactions Chapter 2 discusses the electron density features of pnicogen bond between nitrogen as an electrophile and chlorine as a nucleophile from experimental and theoretical charge density analyses of 2-amino-5-nitropyridine and chloroacetic acid complex. The charge transfer nature of pnicogen bonding due to overlap between donor lone pair orbitals of Cl atom and antibonding N-C sigma star orbital has been demonstrated from gas phase NBO calculations. Presence of sigma hole on N atom is further confirmed from 3D deformation maps and electrostatic maps. Topological description from AIM analysis and energy estimation based on EML method proves this interaction to be weak in nature and comparable to the strength of carbon bonding, type II F•••F halogen bonding. Detailed Cambridge Structural Database (CSD) analysis reveals that planar N atoms have the maximum propensity to participate as electrophile in pnicogen bonding. Chapter 3 reports the frequent occurrence of methyl•••methyl hydrophobic interactions in the solid-state from CSD study with a detailed analysis of these interactions in a series of cocrystals of biologically active molecules such as caffeine, theophylline and tetramethylpyrazine using experimental X-ray charge density analysis, variable-temperature crystallography and solidstate NMR. The visualization of accurate electron density distribution in the interaction region reveals that they are stabilized by the minimized electrostatic repulsion and maximized dispersion forces. This chapter further proves methyl•••methyl HI as a group•••group interaction with a pronounced torsional vibration for the hydrophobic methyl groups which leads to a significant entropic contribution towards its stability. A characteristic C-13 ssNMR up-field chemical shift was found to be associated with these methyl•••methyl interactions in the crystal state. Part B: Correlation of Molecular Conformation with Packing Chapter 4 discusses a new type of polymorphism called hybridized induced polymorphism in connection with the unusual phenomenon of the formation of kinetic form as against the thermodynamic form on slow cooling of boiling aqueous solution of diuretic drug Acetazolamide. Experimental charge density analysis aided with ab initio calculations have investigated the local electron density at the amino region of both polymorphs. A series of crystallization experiments of AZM in aqueous medium were conducted. The boiling solution was ramped down at different rates of cooling; rapid cooling in liquid nitrogen, ambient cooling to room temperature, controlled cooling at (10°C/hr, 7°C/hr and 5°C/hr) to room temperature. PXRD analysis reveals the kinetic form occurs only when the cooling rate is quite slow (7°C/hr and 5°C/hr). The occurrence of both polymorphs from aqueous solution of AZM under different crystallization conditions is rationalized in terms of hybridization induced polymorphism. Chapter 5 investigates electron density distribution in an overcrowded aromatic molecule, Octachloronaphthalene (OCN) by charge density analysis to unravel several unexplored factors responsible for steric hindrance. The topological features of the enigmatic peri interactions contributing to steric overcrowding are qualified and quantified from experimental and theoretical charge density studies. A new facet in the fundamental understanding of peri interactions is revealed by NCI (Non-Covalent Interaction) analysis. The potential role of these interactions in deforming the molecular geometry and subsequent effect on aromaticity are substantiated from NICS (Nuclear Independent Chemical Shift) and QTAIM (Quantum Theory of Atoms in Molecules) calculations. The eye-catching dissimilarity in the out-of-plane twisting of OCN renders the molecule in an asymmetric geometry in the crystalline phase as compared to symmetric geometry in the optimized solvated phase. This is uniquely characterised by their molecular electrostatic potential (MESP) respectively and is explained in terms of conflict between two opposing forces- peri interactions and symbiotic intermolecular Cl•••Cl and Cl••• contacts.

This chapter focuses on microwave and infrared spectroscopic investigations on molecular complexes formed in a supersonic beam, typically at 3 K. These complexes are bound by intermolecular forces that were historically classified as ‘van der Waals forces’ and ‘hydrogen bonding’. As these complexes are investigated at very low T and P, isolated from solvent or lattice effects, intermolecular interactions can be accurately probed. For this same reason, what is learned from molecular complexes in the gas phase may not be directly relevant to the condensed phase, a solution or a crystal. However, comparison of the structure of molecular complexes with that found in the condensed phase has helped in enhancing our fundamental understanding of intermolecular forces. We discuss two specific examples, the phenylacetylene–water complex and 1,2-ethanediol or ethylene glycol, and show how the combination of various spectroscopic and theoretical techniques have been applied over the last decade to unravel the intricacies of inter/intramolecular hydrogen bonds. Intermolecular bonds, involving other elements in the periodic table, in particular, halogen and carbon, are discussed as well. Recent spectroscopic confirmation of a pnicogen bond and nπ* interaction are also pointed out.