摘要:

Potential surfaces for the three lowest-lying electronic states of C2H (12A′, 22A′ and 12A″) calculated by means of theab initioMRD-CI method are transformed into their adiabatic counterparts. Three simple approaches for determination of diabatic transformation are investigated. Diabatic potentials and electronic transition moments obtained as functions of the C-C bond length and the bond angle represent a suitable basis for the calculation of vibronic energy levels and probabilities for transitions between them.

ISSN:0026-8976    

DOI:10.1080/00268979000102061

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A purelyab initiostudy of the vibronic structure of the X2Σ+, A2Π system of C2H is presented. An approach is developed for a simultaneous treatment of three electronic states coupled via the bending and C-C stretching vibrations. On the basis of the results of the present calculations, it is possible to reliably interpret previous experimental findings.

ISSN:0026-8976    

DOI:10.1080/00268979000102071

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The equilibrium distribution functions of free charged particles and those bound in molecules are calculated for a plasma-molecular system restricted by two parallel planes, taking into account all terms of the multipole expansion of the molecular fields. These distributions are used to find the inhomogeneous densities of free charged and neutral particles, the average molecular dipole moments and the total dipole moment of the system. In the case where particles approach the boundary of the system, the expressions obtained can diverge. The removal of the divergences is discussed. For a molecular subsystem modulated by rigidity coupled pairs of charged particles or an ensemble of harmonic atomic oscillators, the equilibrium distributions are investigated in the dipole approximation. The molecules contained in a purely molecular system or a plasmamolecular system with a hydrogen-like plasma subsystem are shown to have zero average dipole moments. For such systems the non-zero average dipole moments of molecules are calculated beyond the dipole approximation. The influence of the boundaries is shown to be similar to some effective field. If the media adjacent to the layer are similar, this effective field repels (attracts) the molecules from (to) the boundary planes if the dielectric permittivity of the molecular subsystem is greater (smaller) than the dielectric permittivity of the external media. The anisotropic equilibrium distributions of rigid dipoles are obtained and the dominant spatial orientations of dipoles are studied. The oscillator eigenfrequency shifts and the mean-square displacements of the oscillators are found for the system of harmonic atomic oscillators. Detailed numerical analyses are performed both for inhomogeneous spatial distributions of molecules and charged particles, and for anisotropic orientational distributions of rigid dipoles in molecular and plasma-molecular layers.

ISSN:0026-8976    

DOI:10.1080/00268979000102081

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

ISSN:0026-8976    

DOI:10.1080/00268979000102091

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The structure of silica gels prepared by acid or basic hydrolysis of Si(OC2H5)4has been investigated by29Si MAS-NMR. The influence of hydrolysis conditions (pH) and drying temperature is studied by determining the percentages of SiO4local coordination units with two, three and four covalent bonds to neighbouring silica centres. Further information is obtained by monitoring the1H-29Si crosspolarization dynamics; an oscillatory component of the magnetization transfer is observed. The cross-relaxation rate 1/TSiHand the spin-diffusion rate 1/THHare discussed in terms of current models of the sol-gel process, in which the structure of the resulting gel is influenced by the kinetics of both the hydrolysis and subsequent condensation reactions.

ISSN:0026-8976    

DOI:10.1080/00268979000102101

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Expressions with no disposable parameters are derived for the self-diffusion coefficientD, shear viscosity, η and thermal conductivity κ of simple liquids in terms of the frequency-transform moments of the relevant time correlation function in the Green-Kubo formulae. We use information theory to derive analytic expressions for these transport coefficients. We also use a Mori-series expansion of the frequency transform of the time correlation function to obtain other estimates of the transport coefficients, including exponential, hyperbolicsecant and Gaussian memory functions, and also geometric-mean closures. Simulations are performed on Lennard-Jones fluid states to calculate both the appropriate frequency moments and also the transport coefficients directly during the same computation. Comparison of the various analytic expressions with the molecular-dynamics simulation values for the transport coefficients reveals that the best agreement is found with one of the analytic memoryfunction closures (the precise form depending on the region of the phase diagram and also on the order of the moment expansion considered). The geometric-mean and low-order information-theory expansions have the advantage, however, that they show fair agreement over thewholephase diagram.

ISSN:0026-8976    

DOI:10.1080/00268979000102111

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The Lebwohl-Lasher model of a liquid crystal is examined in a slab geometry, with free top and bottom surfaces, using standard molecular-field and a new Bethe-type approximation that includes some effects of correlations. The results are compared with computer simulations for a range of slab widthsL.We find that the approximate treatments, while predicting a nematic-isotropic transition that is too strongly first-order, are in semiquantitative agreement with the simulation results. The results of the approximate treatments show that the Kelvin equation for the shift in transition temperature due to confinement is accurate forL≥ 64 layers. For largeLa wetting film of the disordered, isotropic phase intrudes between each surface and the nematic phase. The film thickness, evaluated at the transition in the slab, increases as lnLin accord with the theory of complete wetting for systems with short-ranged forces.

ISSN:0026-8976    

DOI:10.1080/00268979000102121

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Potential functions comprising a sum of two- and three-body terms have been obtained that reproduce the lattice energy, lattice constant, the three elastic constants and the Raman frequency of the diamond lattices of carbon, silicon and germanium. These potentials give the sc, bcc, fcc and hcp lattices of these elements a higher energy. The carbon potential gives a graphite lattice more stable than the diamond lattice, although the stability is overestimated. For silicon and germanium, in contrast, the graphitic structure is significantly less stable than the diamond structure.

ISSN:0026-8976    

DOI:10.1080/00268979000102131

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We employ a recently developed refinement of the computer-simulation algorithm of Edberg, Evans and Morriss to study the rheology of liquid benzene. This is the first simulation study of the rheology of oblate, or disc-like, molecules. We study shear-induced molecular order (shear birefringence) and molecular rotation (the Born effect) and their effect on the shear viscosity of liquid benzene. Our data clearly show that in the linear regime the benzene molecules preferentially align their sixfold rotation axes in the shear plane, at 135° to the streamlines. In this same regime the average angular velocity of the benzene molecules is equal to half the strain rate (ωz= 1/2∂ux/∂y). As the strain rate increases, the fluid at first shear-thins, with the average angular velocity decreasing and the alignment angle decreasing from 135° towards 90°. At very high strain rates the fluid shear thickens, the alignment angle becomes quite close to 90° (so that the plane of the benzene ring is nearly parallel to the streamlines), but the average intrinsic angular velocity increases, again approaching the Born value 1/2∂ux/∂y.

ISSN:0026-8976    

DOI:10.1080/00268979000102141

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We present a comparative analysis of the PISA and RAM models using Monte Carlo (MC) simulations as references. The PISA and RAM models have in common a factorization of the pair distribution into an angular and a radial part. For the angular part both the PISA and RAM models use a Boltzmann weighting. First, we show that this factorization itself and the Boltzmann weighting are satisfactory approximations in a wide range of density and temperature conditions. Indeed, the Boltzmann weighting yields very good average energies (angular average). The chief approximation of the PISA method consists of taking a Heaviside step function for the distribution of the centres of mass. In the RAM theory the distribution of the centres of mass in the molecular fluid is supposed to be the same as the distribution of a spherically symmetric reference fluid. The RAM hypothesis and the PISA approximation are tested and discussed for several molecular liquids (nitrogen, bromine, carbon tetrachloride, benzene, andcis- andtrans-decalin). We show that the RAM theory is unable to give a reliable radial distribution function except for systems studied under favourable conditions (nitrogen and bromine at high temperature). The agreement between MC and RAM radial distribution functions is particularly poor for highly non-spherical molecules liketrans-decalin. The quality of the PISA approximation of the radial distribution function is also discussed in terms of molecular shape. Finally, we show that the more sophisticated RAM theory does not necessarily yield better internal energies than the simplest but very efficient PISA model.

ISSN:0026-8976    

DOI:10.1080/00268979000102151

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor