L. M. Babkov

Polymorphism and conformational mobility of mesogens

Abstract The results of complex investigations into polymorphism and mesomorphic states in the homologous series of alkylbenzoic, alkoxybenzoic, and alkylcyclohexanecarboxylic acids, some of their fluorinated derivatives, and 4′-cyano-4′-n-alkoxybiphenyls, where n is the number of carbon atoms in the alkyl and fluoroalkyl radicals, have been discussed. The methods of IR absorption spectroscopy (theoretical and experimental), differential thermal analysis, X-ray diffraction analysis, and polarization microscopy were used in this work. The specimens were investigated over a wide temperature range (100–500 K). The spectra were recorded in the range 33–4000 cm−1. Several polymorphic crystalline modifications and mesophases were revealed in the specimens investigated, and conditions of their realization were elucidated. Patterns of molecular packing in crystals of the homologous series of compounds investigated were proposed. Both changes in the conformational composition at increasing temperature and the presence of several conformers realized simultaneously in the same specimens over certain temperature intervals were detected. The influence of AR fluorination and of H-bonds on mesophase formation was determined.

Hydrogen bonding, IR spectrum, and the structure of methyl-β-D-glucopyranoside

A theoretical study of the structure and vibrational spectrum of methyl-β-D-glucopyranoside is performed with allowance for the hydrogen bond effect on them. At the density functional theory level with the use of the B3LYP functional in the 6–31G(d) basis set the structural dynamic models of a free molecule of methyl-β-D-glucopyranoside and its simplest complexes with hydrogen bonding in the form of dimers with different structures are constructed. Energies are minimized; structures, electro optical parameters, force constants, and normal vibrational frequencies in the harmonic approximation and their intensities in IR spectra are calculated; the hydrogen bond energy is estimated. Based on the calculation, the conclusions are drawn about the structure of the methyl-β-D-glucopyranoside sample, the formation and interpretation of its IR spectrum, and the possibilities of the used density functional theory method.

Structure and Conformational Mobility of 4′‐Pentyl‐4‐Cyanobiphenyl from IR Spectroscopic Data

The structure and conformational mobility of 4′‐pentyl‐4‐cyanobiphenyl in the liquid and liquid‐crystal states were investigated by means of IR spectroscopy (experiment and theory). IR absorption spectra in the frequency range 400–4000 cm-1 were measured in the temperature range 26–150°. Theoretical simulation of the IR spectra was performed using the fragment approach. A comparison of theoretical and experimental spectra showed that the examined samples were mixtures of conformers; characteristics of the most probable conformers are given. Temperature‐related changes in the spectrum are due to the conformational mobility of the molecules, which is not high in the temperature range considered.

IR Spectra, Polymorphism, and Intermolecular Interactions in Carboxylic Acids

This paper summarizes the results of our studies on the structural aspects of polymorphism and mesomorphous states in long-chain aliphatic compounds represented by the homologous series of alkyl-, alkoxybenzoic, and alkylcyclohexanecarboxylic acids and their completely or partially fluorinated analogs. The studies were carried out by vibrational IR spectroscopy (experiment, theory), calorimetry, polarization microscopy, and X-ray diffractometry. Conformational polymorphs have been found for the compounds, which differ in the bending of alkyl and fluoroalkyl radicals and in rotation of carboxyl groups and phenyl rings during phase transitions. Structural rearrangements and dynamic changes take place inside the nuclei of H-complexes. The dimers are partially dissociated, and open associates, chain associates, and monomers differing in their conformational composition are formed.

Quantum mechanical modeling of the 2-biphenylmethanol molecule

The quantum mechanical investigation of 2-biphenylmethanol molecule is represented in this work. The calculations of the structure and IR absorption spectra have been carried out using B3LYP method and 6-31G (d) basis set. Two conformers of the molecule were found and then they were compared with the structure calculated by LEV-100.

Conformational Mobility and Phase Transitions in Alkyloxybenzoate of Cholesterol

Abstract The structural particularities of polymorphic modifications and cholesterol liquid crystal phase have been investigated by theoretical and experimental IR spectroscopy methods. The spectra were recorded in the interval of temperatures 293–523 K within the range of 600–3600 cm−1. The frequencies of normal modes and intensities in the IR spectra have been calculated for the alkyloxybenzoate of cholesterol conformers, which may be realized under certain temperatures. Comparison of results of theoretical modelling and recorded spectra confirmed suggestion about conformational mobility basically defined with changing of steroid kennel orientation comparatively stayed part of molecule in the different phase state.

Conformational Polymorphism of Fluorinated Derivatives of Alcylcyclohexanecarboxylic Acids

Abstract The results of investigations of polymorphism and mesomorphic states of fluorinated derivatives of alkylcyclohexanecaroxylic acids nFACHCA (where n is the number of carbon atoms in the fluoroalkil radicals) and new results for n=4, 7 have been discussed. The methods of vibrational spectroscopy (experimental, theoretical modeling) have been used. The samples have been investigated in the 290–500 K temperature range. The spectra have been recoded in the range of 33–4000 cm−1. The frequencies of normal modes and intensities of bonds of IR spectra have been calculated for conformers of H-complexes with various orientations of fluoroalkyl radical and carboxylic group relative to the rest part of the molecular complex. The polymorphism of the nFACHCA is conformational. The influence of H-bonds on dynamic of H-complex, polymorphic crystalline modification and mesophase formation were determined. The transitions solid crystal- liquid crystal-isotropic liquid and solution are accompanied by H-bonds system reconstruction: the dimmers, associates, chain associates and monomers have been appeared.

Vibrational spectra and structure of 4-cyano-4′-pentalkoxybiphenyl in different physical states

The polymorphism, conformation mobility and structure of 4-cyano-4′-pentalkoxybiphenyl (5OCB) in different physical states are studied by IR spectroscopy. The spectra were measured in the frequency range 400–4000 cm−1 at temperatures from 300 to 350 K. The IR spectra of 5OCB are modeled using the concept of conformational mobility of these molecules. An analysis of the experimental and theoretical spectra reveals absorption bands whose spectroscopic parameters are sensitive to variations of temperature (experiment) and conformation (theory); a relationship between these changes is established. It is concluded that the polymorphism of 5OCB is of conformation type. In the solid crystalline state, 5OCB molecules have conformations with a planar biphenyl fragment; the angle of orientation of the plane of the carbon framework of the alkyl radical relative to the biphenyl fragment decreases as the temperature increases from 35° in the solid crystalline state to 10° in the liquid crystalline and isotropic liquid states. In both of these states the biphenyl fragment becomes nonplanar. The angle between the phenyl rings is up to 30°.

Modeling of 2,3-di-O-nitro-methyl-β-D-glucopyranoside taking into account the H-bonding using DFT method and interpretation of the IR spectrum of the sample

Structural-dynamic models of 2,3-di-O-nitro-methyl-β-D-glucopyranoside and its H-complexes have been obtained using density functional method. B3LYP hybrid functional and 6-31G (d) basis have been applied. Energies, structures, dipole moments, polarizabilities, frequencies of normal modes in harmonic approximation and IR intensities have been obtained. IR spectra of the complexes of 2,3-di-O-nitro-methyl-β-D-glucopyranoside and ethanol has been interpreted in 600-3700 cm–1 region. Taking into account the results of simulation we have interpreted IR spectrum of the sample in region 600-3700 cm-1, and determined the structure of the sample.

Vibrational spectra and structural-dynamical models of the 2-,3-and 4-biphenylmethanoles

In connection with experimental IR spectra of the 2-, 3-, and 4-biphenylmethanols in solid crystal state the presence of the hydrogen bond in their samples was found. Using the density functional theory (B3LYP/6-31G*), the following parameters of the compounds mentioned above and of their H-bond complexes, which are formed in solid crystal phase have been computed: the energies, structures, dipole moments, polarizabilities, frequencies of the normal modes in harmonic approximation, IR intensities, Raman activities. On the basis of analysis of experimental spectra and modeling results the issues about crystal structure peculiarities of 2-, 3-, and 4-biphenylmethanols and about the influence of H-bond on the dynamics of their molecular systems were made.