Molecular ordering of Fluoro-Nematogen in a dielectric medium at phase-transition temperature: a statistical–mechanical approach based on computer simulation

A molecular-ordering study of p-phenylene-4-methoxy benzoyl 4-trifluoromethylbenzoate (FLUORO) in a dielectric medium (benzene) has been carried out on the basis of statistical mechanics and computer simulation. 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 the phase-transition temperature (506?K), using the MB-formula. The flexibility of various interacting configurations was studied in terms of variation of probability due to departure from the most probable configuration. It has been observed that in dielectric medium the energies/probabilities are redistributed, and there is considerable rise in the probability of interaction, although the order of preference remains the same. The results are discussed in the light of experimental observations.     

Ordering of p‐n‐alkylbenzoic acids (nBAC) having four,five, and six alkyl chain carbon atoms – a computational analysis

A computational analysis of ordering in p‐n‐alkylbenzoic acids, having 4 (4BAC), 5 (5BAC), and 6 (6BAC) alkyl chain carbon atoms, has been carried out based on quantum mechanics and intermolecular forces. The evaluation of atomic charge and dipole moment at each atomic centre has been carried out through an all‐valance electron (CNDO/2) method. The 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 at room temperature, nematic‐isotropic transition temperature and above transition temperature using the Maxwell‐Boltzmann formula. A comparative picture of molecular parameters like total energy, binding energy and total dipole moment has been given. A model has been developed to describe the nematogenicity of these acids in terms of their relative order with molecular parameter introduced in this article.     

Molecular ordering in a bipolar nematogenic cyanobiphenyl – a computer simulation approach

The molecular ordering of 4-(6-hydroxypropyloxy)-4′-cyanobiphenyl (H3CBP), a bipolar nematogen, has been carried out at room temperature (300 K) with respect to translatory and orientational motions. The complete neglect differential overlap (CNDO/2) method has been employed to compute the net atomic charge and atomic dipole moment at each atomic center. The modified Rayleigh–Schrodinger perturbation theory 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 interaction energy values obtained through these computations were used to calculate the probability of each configuration at room temperature using the Maxwell–Boltzmann formula. On the basis of stacking, in-plane, and terminal interactions energy calculations, all possible geometrical arrangements of the molecular pair have been considered. Further, an attempt has been made to develop a model for liquid crystallinity based on probability of dimer complexes.     

Thermodynamic stability and phase behaviour of a liquid crystalline material: a theoretical model at molecular level

This article describes the thermodynamic stability and phase behaviour of a liquid crystalline material p-n-hexyloxybenzylidene-p-toluidine (6OBT) at a molecular level. The atomic net charge and dipole moment at each atomic centre have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh–Schrodinger perturbation method along with multicentred-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for the short-range interactions. The total interaction energy values obtained through these computations have been used to calculate the probability of each configuration at room temperature (300?K), nematic–isotropic transition temperature (346.9?K) and above transition temperature (400?K) using the Maxwell–Boltzmann formula. Further, the Helmholtz free energy and entropy of each configuration has been computed during the different modes of interactions. An attempt has been made to understand the phase behaviour and stability of the molecule based on thermodynamic parameters introduced in this article.     

Theoretical study of molecular ordering in liquid crystals with the help of intermolecular interaction energy calculations

A theoretical analysis has been carried out to determine the molecular alignment of some nematic liquid crystals like DPAB (4, 4'-di-n-propoxy-azoxybenzene), EPAB (ethyl para-azoxybenzoate) and PAA (para-azoxyanisole). CNDO/2 method has been employed to evaluate the net atomic charge and atomic dipole components at each atomic centre of the molecules. Configurational energy has been computed using a modified Rayleigh-Schrodinger perturbation method at an interval of 1 Å in translation and 10° in rotation between a pair of DPAB molecules. On the basis of stacking, in-plane and terminal interaction energy calculations, all possible geometrical arrangements between a molecular pair have been considered. A comparative picture of molecular parameters like total energy, binding energy, total dipole moment etc., has been obtained. An attempt has been made to explain the nematogenic behaviour of liquid crystal and thereby develop a molecular model for liquid crystallinity.     

Molecular Ordering of a Nematogen at Phase Transition Temperature - A Theoretical Study

Molecular ordering in 4-cyano-4'- n -propylbiphenyl ( CB3 ), a nematic liquid crystal, has been studied with the help of intermolecular interaction energy calculations. The CNDO / 2 method has been employed to compute the net atomic charge and atomic dipole moment at each atomic centre. 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 interaction energy values obtained through these computations were used to calculate the probability of each configuration at the phase transition temperature using Maxwell-Boltzmann formula. All possible geometrical arrangements between molecular pair have been considered during stacking, in-plane and terminal interactions. An attempt has been made to identify the most probable configuration at the phase transition temperature during stacking, in-plane and terminal interactions. Results obtained have been discussed in the light of experimental as well as other theoretical observations.     

Molecular alignment of biphenyl derivatives -a computational analysis

Molecular alignment of some biphenyl derivatives likes 4'-Nitro-2-biphenylamine (NBPA), 4-Acetyl-3'-chlorobiphenyl (ACBP) and 4-Acetyl-3'-florobiphenyl (AFBP) has been examined. The CNDO/2 method has been employed to compute the net atomic charge and atomic dipole-moment components at each atomic centre. Configurational energy has been computed using a modified Rayleigh- Schrodinger perturbation method at an interval of 0.1 A° in translation and 1° in rotation, and corresponding probabilities have been calculated using MB-statistics. A comparative study of molecular parameters like the total energy, the binding energy and the total dipole moment etc., has been made. On the basis of the stacking and the in-plane interaction energy calculations, all the possible geometrical arrangements between molecular pairing have been obtained. The investigation suggests that a strong intermolecular interaction energy between a pair of NBPA molecules, and the specific minimum energy configuration, determines the alignment of molecules with respect to one another. An attempt has been made to correlate the liquid crystalline properties exhibited by this class of molecules, and thereby develop a molecular model for the liquid crystallinity.     

Phase organization in laterally substituted dicyclohexylethane derivatives—A molecular model for smectogens

The present article deals with the configurational probabilities of smectogens 1, 2-bis (4-pentylcyclohexyl) ethan-1-ol (PCHE1) and 1, 2-bis (4-pentylcyclohexyl) ethan-1-one (PCHE2) with respect to translational and orientational motions. The complete neglect differential overlap (CNDO/2) method has been employed to compute the net atomic charge and atomic dipole moment components at each atomic center. The modified Rayleigh-Schrodinger perturbation theory along with multicentered-multipole expansion method has been employed to evaluate the 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 have been used to calculate the configurational probability at smectic B-isotropic transition temperature using the Maxwell-Boltzmann formula. Molecular arrangements inside a bulk of materials and smectic behavior of the compounds in terms of their relative order have been discussed. Further, a model has been developed at molecular level to understand the effect of translational/rotational rigidity on phase organization and relative flexibility of one configuration over the other.     

Nonadiabatic couplings and charge transfer study in H + CS+ collision using time-dependent quantum dynamics

Experiments have reported the high stability of HCS⁺ ion and inhibit to decompose over the range of collision energies. In this study, the various energy transfer channels of atomic H collision with CS⁺ molecular ion has been performed by ab initio computations at the multireference configuration interaction/aug-cc-pVQZ level of theory. The ground and several low-lying excited electronic state potential energy surfaces in three different molecular orientations, namely, two collinear configurations with, (1) H approaching the S atom (γ = 0°), (2) H approaching the C atom (γ = 180°) and one perpendicular configuration, (3) H approaching the centre of mass of CS (γ = 90°) with the diatom fixed at the equilibrium bond length, have been obtained. Nonadiabatic effects with Landau–Zener coupling leading to avoided crossings are observed between the ground- and the first-excited states in γ = 90° orientation, and also between the first- and second-excited states in γ = 180° orientation. Quantum dynamics have been performed to study the charge transfer using time-dependent wave packet method on the diabatic potential energy surfaces. The probability of charge transfer is found to be highest with 42% in γ = 180°. The high charge transfer probability result in the formation of H⁺ + CS channel which ascertains the high stability of HCS⁺ ion.     

Theoretical analysis on phase behaviour of a liquid crystalline material – effect of molecular motions

Molecular structure, and phase behaviour of 2-Cyano-N-[4-(4-n-pentyloxybenzoyloxy)-benzylidene] aniline (CPBBA) has been reported with respect to translational and orientational motions. The atomic net charge and dipole moment components at each atomic centre have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh–Schrodinger perturbation theory along with multicentered–multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for short-range interactions. The interaction energy values obtained through these computations have been used as input to calculate the configurational probability at room temperature (300 K), and nematic–isotropic transition temperature (396.5 K). On the basis of stacking, in-plane, and terminal interaction energy calculations, all possible geometrical arrangements between the molecular pairs have been considered. Molecular arrangements inside a bulk of materials have been discussed in terms of their relative order. Further, translational rigidity parameter has been estimated as a function of temperature to understand the phase behaviour of the compound. The present model is helpful to understand the effect of molecular motions on ordering, and phase behaviour of the mesogenic compounds.     

Self-interstitial configuration in molybdenum studied by modified analytical embedded atom method

The stability of various atomic configurations containing a self-interstitial atom (SIA) in a model representing Mo has been investigated using the modified analytical embedded atom method (MAEAM). The lattice relaxations are treated with the molecular dynamics (MD) simulation at absolute zero of temperature. Six relatively stable self-interstitial configurations and formation energies have been described and calculated. The results indicate that the [111] dumbbell interstitial S₁₁₁ has the lowest formation energy, and in ascending order, the sequence of the configurations is predicted to be S₁₁₁, C, S₁₁₀, T, S₀₀₁ and O. From relaxed displacement field up to the fifth-NN atoms of six configurations, we know that the relaxed displacements depend not only on separation distances of the NN atoms from the defect centre but also strongly on the direction of the connected line between the NN atoms and the defect centre. The equilibrium distances between two nearest atoms in the core of the S₁₁₁, C, S₁₁₀, T, S₀₀₁ and O configurations are 0.72a, 0.72a, 0.71a, 0.72a, 0.70a and 0.70a, respectively.      

Effect of substituent on UV–visible absorption and photostability of liquid crystals: DFT study

The electronic transitions in the ultraviolet–visible (UV–Vis) range of two nematogens, namely 4′-cyanophenyl-4-n-pentylbenzoate and 4′-cyanophenyl-4-n-pentoxybenzoate, have been studied. The UV–Vis and circular dichroism spectra of these molecules have been simulated using the TDDFT/B3LYP/6-31+G(d) method. Mulliken atomic charges for each molecule have been compared with Loewdin atomic charges to analyze the molecular charge distribution and phase stability. The highest occupied molecular orbital and lowest unoccupied molecular orbital energies corresponding to the electronic transitions in the UV–Vis range have been reported. The excited states have been calculated via configuration interaction singles with semi-empirical Hamiltonian ZINDO (Zerner's intermediate neglect of differential overlap method). Further, the effect of substituent on ultraviolet absorption and photostability of the molecules has been discussed. The photostability of the molecules has been investigated in order to understand the application and operation with ultraviolet and visible light regions.     

Physical characterisation of 4′-butyl-4-heptyl- bicyclohexyl-4-carbonitrile

We report measurements of birefringence, bend elastic constant, principal dielectric constants and conductivities as functions of temperature in the nematic phase of 4′-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). The compound exhibits a small birefringence and a large negative dielectric anisotropy. Surprisingly, it shows a negative conductivity anisotropy, which probably owes its origin to the structural details of the molecules. The orientational order parameters (S) are estimated from both birefringence and dielectric anisotropy, and they agree well in the entire nematic range. The experimental results are compared with those on CCH7, which has a similar core structure but with a longitudinal dipole moment, in place of the transverse one of CCN-47.     

The influence of collective effects on the propagation of electromagnetic radiation in dense ultracold atomic ensembles

Based on the already-developed general theory (I.M. Sokolov, D.V. Kupriyanov, and M.D. Havey, JETP 112 (2), 246 (2011)), we have studied the spatial distribution of excited atoms and of the atomic polarization that a weak monochromatic field creates in a dense ultracold atomic medium. We show that, in the case of a homogeneous random spatial distribution of atoms, the amplitude of atomic polarization averaged over spatial configurations decreases outside boundary regions according to an exponential law, while its phase linearly increases. Based on this, we have numerically determined the extinction coefficient and the light wavelength in the medium, as well as its dielectric permittivity. The dispersion of the permittivity at different concentrations has been studied. We show that, for dense clouds, the real part of the dielectric permittivity acquires negative values in a certain frequency range. Based on the calculation of the spatial distribution of excited atoms, we have analyzed the character of the transfer and trapping of quasi-resonant radiation in atomic clouds of differing density.     

镍基单晶高温合金γ/γ′（001）相界面上原子构型的分子动力学研究

用分子动力学方法研究了镍基单晶高温合金γ/γ′（001）相界面上三种各具特征的原子堆垛结构. 能量学计算发现，存在最优构型，动力学模拟显示不同构型的界面弛豫后，在相界面上都“成对”出现刃型错配位错. 相关计算表明体系能量、界面形成能及弛豫能都依赖于界面原子堆垛特征，而几何特征则具共性，即不同原子构型的界面具有同一的应力释放模式.     

Spontaneous regeneration of an atomically sharp graphene/graphane interface under thermal disordering

The smearing of the graphene/graphane interface owing to the thermally activated migration of hydrogen atoms has been investigated by the molecular dynamics method. Contrary to expectations, it has been found that the fast spontaneous regeneration of this interface occurs even at a sufficiently high temperature T ≈ 1500 K; as a result, the average width of the disordered region does not exceed the length of a C-C bond; i.e., the interface remains almost atomically sharp. The cause of this effect appears to be the specific shape of the potential relief of the system, namely, the significant difference between the heights of the energy barriers for the direct and inverse migrations of hydrogen. A simple model that makes it possible to obtain the temperature dependence of the equilibrium distribution function of typical atomic configurations, to estimate the typical time of establishing the equilibrium state, and thereby to quantitatively describe the results of the computer experiment is presented.     

Effect of mutual configuration between molecular orientation and atomic orientation in the oriented Ar (3P2)+oriented CF3H reaction

The steric effect in the dissociative energy transfer reaction of Ar (3P2,MJ=2)+CF3H has been observed as a function of the mutual configurations between the molecular orientation and the atomic orientation in the collision frame. The reactivity is significantly changed depending on the mutual configuration between the molecular orientation and the atomic orientation. Especially, the molecular alignment dependence is dramatically changed depending on the configuration of the atomic alignment.     

AlOH(CS，X1A′)分子的结构与势能函数

使用MP4方法,在6-311G(3df,3pd)基组水平上对AlOH(C_S,X¹A′)基态分子进行了几何优化,得到了它的平衡几何构型和力常数.根据原子分子反应静力学原理得到AlOH分子的电子状态和可能的离解极限.应用多体展式理论方法推导出了AlOH基态分子的解析势能函数. 关键词： AlOH 分子结构 解析势能函数 多体展式理论