摘要:

Theories of certain electromagnetic effects, such as chiral phenomena in fluids and crystals and gyrotropic birefringence in antiferromagnetic crystals, require the inclusion of electric quadrupole contributions for a full description of the effect. In a number of these theories the electric quadrupole moment is defined to be traceless, as indeed is the general practice for such moments in nuclear physics. It is shown that, when the traceless quadrupole moment is used in the derivation of the wave equation that describes light propagation through an optically active uniaxial medium, this equation and properties derived from it, in particular refractive index, depend in general on the arbitrary origin used to specify the quadrupole moment. This is physically unacceptable for an observable property of a substance. By contrast, this defect does not occur if the definition of the primitive quadrupole moment is adopted, namely Σqrαrβ.

ISSN:0026-8976    

DOI:10.1080/002689797171085

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The local and the differential virial theorems proposed by Bader, Ziesche, Holas and March, Nagy and Parr, and Nagy and March are compared. New path-integral forms of the virial theorem are then derived. Using the Harbola-Sahni conjecture for the exchange-correlation potential, another, new approximate form of the local virial theorem is obtained.

ISSN:0026-8976    

DOI:10.1080/002689797171094

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We report results from a series of molecular dynamics simulations designed to study the phase behaviour of model rod-like liquid crystal molecules with different geometries interacting via the Gay-Berne potential. Following the classification of molecular geometry in terms of a multipole expansion in steric asymmetry, two models have been studied in detail: a zigzag model defined as a steric quadrupole and a triangle model defined as a longitudinal steric dipole, and comparison has been made with a cylindrical model. Results from theNVEensemble indicate that the model steric quadrupole delays the temperature of onset of the nematic phase. Extensive simulations in theNPTensemble demonstrate a similar trend in the temperature of onset of the smectic B phase, with a lower temperature of onset observed with the steric quadrupole than the steric dipole. Local antiparallel steric ordering within a layer was observed with the model steric dipole in the crystal B phase but not with the model steric quadrupole. This structure is in agreement with experimental results and with the prediction of the generalized molecular asymmetry model. The steric quadrupole demonstrated a rippled structure through the smectic B phase, increasing in amplitude and wavelength sufficiently to tilt molecules along a wave with respect to the system director as the system was cooled. This structure was almost absent in the final crystal structure simulated. The ensemble also allowed comparison with experiment and agreement, scaled with respect to the single-site Gay-Berne mesogen, was found to be good.

ISSN:0026-8976    

DOI:10.1080/002689797171102

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The associative replica Ornstein-Zernike equations, supplemented by the associative hypernetted chain approximation are used to describe the Smith-Nezbeda model for an associating fluid adsorbed in a disordered hard sphere matrix. These equations have been developed by combining the replica Ornstein-Zernike equations with the Wertheim theory of association in fluids. The pair distribution functions and fraction of unbonded species are calculated. Also investigated is the influence of the association on the equation of state and the chemical potential of the absorbed fluid.

ISSN:0026-8976    

DOI:10.1080/002689797171111

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The intermolecular potential energy surface (PES) for the interaction between the Cl2C2H2molecules has been comprehensively investigated using ab initio methods, aiming to locate the possible stationary points. The calculations were performed with the double zeta plus double polarization (DZ2P), triple zeta plus polarization (TZP) and triple zeta plus double polarization (TZ2P) basis sets, including electron correlation at the second-order Moller-Plesset (MP2) level, and basis set superposition error correction. Six stationary points were located on the PES: a T-shaped form where one chlorine atom is attached to the acetylene triple bond (bπ–σ type), a parallel form, a slipped parallel form, a crossed form and an inclined and a symmetric inverse T-shaped forms, where the van der Waals bond is between one of the H atoms of the acetylene and the Cl–Cl bond. At the MP2/TZ2P//MP2/TZP level of calculation, only the T-shaped and the parallel forms are minimum energy structures, and their stabilization energies are 2·002 and 0·422 kcal mol-1The two inverse T-shaped forms and the slipped parallel form are predicted to be first-order transition states at this level of calculation, and their stabilization energies are 0·709 kcal mol-1for the inclined form, 0·694 kcal mol-1for the symmetric form, and 0·624 kcal mol-1for the slipped parallel form, which suggest that the intermolecular PES is very flat in this region. The crossed form is a second-order transition state, and it is stabilized by 0·390 kcal mol-1. The shifts of the intramolecular frequencies upon complexation are also discussed. The global minimum is the T-shaped bπ–aσ structure, and the geometry, the intermolecular stretching force constant and the charge redistribution on complex formation lead to a classification of the outer (weak) type according to Mulliken. These results show the weakness of this interaction, which is dominated by dispersion forces, characteristic of the complexes between molecules without a permanent electric dipole, such as C2H2and Cl2.

ISSN:0026-8976    

DOI:10.1080/002689797171120

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper summarizes the results of a molecular dynamics study of the clathrate silicate dodecasil 3C with different methane fillings, which covered the influence of cavities on the behaviour of entrapped molecules. The simulations treat all degrees of freedom of methane at room temperature. The dynamics of methane is influenced very little by use of a flexible silicate framework, as compared with use of a rigid one. The calculated infrared and Raman spectra show that two types of molecular environment can be clearly recognized from the vibrational behaviour of the molecules. Also, in the low-frequency region the rigid molecule motions are well differentiated. In small cages the molecules are located near the centres of cages, whereas in large cages the molecules move along the walls of the cavities while their orientation is nearly free.

ISSN:0026-8976    

DOI:10.1080/002689797171139

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The bound and quasibound vibrational states of ozone in the1B2electronic state have been calculated using the stabilization method. The calculation was based on anab initiopotential energy function from the literature. From the topology of this surface, we determine the molecular symmetry group of B2O3to be C2v(M). The parameters of resonances in the 750–2750 cm-1energy range relative to the O2(1Δg) + O2(1D)dissociation limit are given. The effective vibrational constants obtained by fitting the calculated vibrational energy levels are compared with the corresponding experimental values. The agreement is satisfactory and supports the assignment of the Huggins band to the1B2X1A1electronic transition.

ISSN:0026-8976    

DOI:10.1080/002689797171148

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The free jet millimetre wave spectrum of the 2,5-dihydrofuran-argon molecular complex has been investigated in the frequency range 60–78 GHz. Estimates of the geometry and of the van der Waals force constants are given.

ISSN:0026-8976    

DOI:10.1080/002689797171157

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The diffusion of methane in partially dealuminated zeolite Na A has been studied by molecular dynamics using a vibrating framework model at different loadings and with different aluminium content. Methane molecules were represented by Lennard-Jones particles. The computed diffusion coefficients increase with increasing loading and decreasing aluminium content, in agreement with experiment. The mechanism of the diffusive process is discussed and a detailed analysis of the behaviour of methane molecules in partially dealuminated zeolite Na A is reported.

ISSN:0026-8976    

DOI:10.1080/002689797171166

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The nitroxide C9H16NO2(tano) forms channel inclusion compounds with a large range of linear chains or slightly branched molecules. All of them present disorder phenomena of the guest chains in the channels of the structure, but also of the host matrix molecules from one chiral form to the other. The evolution of this disorder as a function of the temperature is accompanied by one or two phase transitions above 100 K. The incoherent quasielastic neutron scattering (IQNS) technique was used to study the motion of the tano molecules for the tano/1-bromohexadecane and the tano/1-bromodecane systems in their low, intermediate and high temperature phases. The experiments were carried out on a polycrystalline sample at the Institut Laue–Langevin in Grenoble using the time-focusing time-of-flight spectrometer IN6 with a temperature ranging from 103 K to 300 K. The motion of the guest chains also was shown. The results are examined in terms of the general model proposed in an earlier IQNS study of other tano/alkane compounds with shorter included chains and the original features associated with the chain length are discussed.

ISSN:0026-8976    

DOI:10.1080/002689797171175

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor