Molecular ordering of 4O.2 compound in a dielectric medium at room temperature‐A theoretical approach

The molecular ordering of N‐(4‐n‐butoxybenzylidene)‐4′‐ethyaniline [4O.2] in a dielectric medium (benzene) has been carried out on the basis of statistical mechanics and intermolecular forces. The atomic net charge and atomic dipole moment at each atomic centre has been evaluated using the CNDO/2 method. The modified Rayleigh‐Schrodinger perturbation theory with multicentered‐multipole expansion method has been employed to evaluate the long‐range intermolecular interactions, while ‘6‐exp’ potential function has been assumed for the short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in a dielectric medium at room temperature (300K) using the MB‐formula. The flexibility of various interacting configuration has been studied in terms of variation of probability due to departure from the most probable configuration. A comparative picture of the molecular parameters like total energy, binding energy and total dipole moment has been given. An attempt has been made to analyse the characteristic features of nematogenic 4O.2 compound in terms of its relative order. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nematic Behavior of 5OCB in a Dielectric Medium at Phase Transition Temperature ‐A Statistical Analysis

A statistical analysis has been carried out to determine the configurationally preference of a pair of 5OCB (4′‐n‐pentyloxy‐4‐biphenylcarbonitrile) molecules with respect to translatory and orientational motions. The CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecule. Modified Rayleigh‐Schrodinger perturbation theory along with multicentered‐multipole expansion method has been employed to evaluate long‐range intermolecular interactions, while a ‘6‐exp’ potential function has been assumed for short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in vacuum and in a dielectric medium at phase transition temperature using Maxwell‐Boltzmann formula. It has been observed that the probabilities are redistributed in dielectric medium and there is considerable rise in the probabilities of interactions, although the order of preference remains the same. On the basis of stacking, in‐plane and terminal interaction energy calculations, all possible geometrical arrangements of the molecular pairs have been considered. The results are discussed in the light of experimental as well as other theoretical observations.     

Role of dielectric medium on mesogenic compounds (BCHs) at phase transition temperature: A comparative computational analysis

A comparative computational analysis on two liquid crystalline disubstituted biphenylcyclohexanes (BCHs) of general formula R‐C₆H₁₀‐C₆H₄‐X with R: C₃H₇; X: H (BCH30) and R: C₅H₁₁; X: CN (BCH5CN) has been carried out on the basis of quantum mechanics and intermolecular forces. The evaluation of atomic charge and dipole moment at each atomic centre has been carried out through an all‐valence electron (CNDO/2) method. The configurational energy has been computed using the Rayleigh‐Schrodinger perturbation method. The total interaction energy values obtained were used to calculate the probability of each configuration in dielectric medium (i.e. non‐interacting and non‐mesogenic solvent, benzene) at room temperature, transition temperature and above transition temperature using the Maxwell‐Boltzmann's formula. The flexibility of various interacting configurations has been studied in terms of variation of probability due to small departures from the most probable configurations. Further, the various possible geometrical arrangements between a molecular pair during the different modes of interactions have been considered. An attempt has been made to develop a new and interesting model for mesogenic compounds in a dielectric medium. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectroscopic investigation of strain induced by compositional variation in bulk InxGa1–xAs crystal grown by MCZM method

Raman scattering (RS), photoluminescence (PL) and energy dispersive X‐ray (EDX) experiments have been carried out to investigate residual strain and hence to understand breakage issue in bulk In_x Ga_1–x As crystal grown by multi component zone melting (MCZM) method. It is found from a comparison that there is a large discrepancy among the RS, PL and EDX results due to the strain induced by compositional variation in the crystal. The strain induced changes in TO_GaAs and PL peak positions are found to be 4.04 cm^–1 and 0.097 eV, respectively, for the variation of composition from 0.06 to 0.29 from the seed‐end to the tail‐end of the crystal. By assuming a simple one‐dimensional strain distribution, the strain value corresponding to 4.04 cm^–1/ 0.097 eV can be obtained of the order of 10^–2, which is large enough for understanding the breakage issue in the crystal investigated here. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)