摘要:

We report the temperatureTdependence of the proton Zeeman relaxation rateRin solidt-butylbenzene at 8·50, 22·5 and 53·0MHz. The relaxation is caused by the modulation of the spin-spin dipole-dipole interactions by the reorientingt-butyl groups and their constituent methyl groups. It is shown that even if the phenyl group reorients, it makes a very small contribution to the observed relaxation rate. The observedRversusTis very complicated and this is unusual for such simple solids ast-butylbenzene. The data are fitted with several models for the dynamical properties of thet-butyl group and its constituent methyl groups, each assuming a particular symmetry imposed by the crystal structure.

ISSN:0026-8976    

DOI:10.1080/00268979000100311

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The dipolar couplingsDijpreviously measured for tetrahydrofuran (THF) in nematic Phase IV have been analysed in order to obtain information about the puckering amplitude and pseudorotation potential of this molecule. The effective ordering matrix (EOM) method and the ELS theoretical model have been applied by using a hindered pseudorotation potential of typeV2cos 2φ +V4cos 4φ. The optimized value for the equilibrium puckering amplitude of 0·39 Å is close to the value of 0·38 Å determined by electron diffraction. The actual pseudorotation potential for THF cannot be determined because the surfaces σ(V2,V4) for the rms deviations between observed and calculatedDijvalues show flat valleys instead of local minima. Notwithstanding, the results obtained by means of the ELS model, which allows for the dependence of orientational ordering on molecular shape, are more consistent with the spectroscopic result of almost free pseudorotation for the THF than those provided by the EOM method.

ISSN:0026-8976    

DOI:10.1080/00268979000100321

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Two infrared band systems, centred near 846 nm and 1060 nm in both MnH and MnD have been rotationally analysed and shown to have a common lower state. The electronic transitions lie within the quintet manifold and are here designatedc5Σ-a5Σ andb5Πi-a5Σ for the 846 and 1060 nm systems, respectively. In the 846 nm system in MnH all 10 main branches have been found in the 0-0 and 1-1 bands, while in MnD the data are complete only for 0-0. Satellite branches have been identified in the 1060 nm system and all spin and Λ-type doublings have been established. The number of assigned 0-0 branches in the 1060 nm system is 35 for MnH and 37 for MnD. Molecular constants have been determined for all three states involved. Λ-doubling diagrams are presented forb5Πistate withv= 1, 2 levels in MnH and with thev= 2 level in MnD. Local perturbations inc5Σ (v= 1) in MnH are suspected to originate from thev= 3 level ofb5Πi.

ISSN:0026-8976    

DOI:10.1080/00268979000100331

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In order to determine the vapour liquid equilibrium of a pure fluid, the liquid and the vapour branch of the isotherms in the chemical potential μ vs pressurep-diagram, are constructed explicitly. The liquid branch is obtained by molecular dynamics simulations in anNpT-ensemble into which test particles are inserted to calculate the chemical potential. The vapour branch is obtained at lower temperatures by using the second virial coefficient, at higher temperatures it is determined again by simulations. As an example the two-centre Lennard-Jones fluid with elongationL= 0·505 is considered at temperatures ranging from 0·69 to 0·92 of the estimated critical temperature. As expected, the inaccuracies of the liquid chemical potential increase with decreasing temperature as a consequence of the increasing saturated density. The uncertainties in μ/RTrange from 0·02 at the highest to 0·10 at the lowest temperature which creates an uncertainty in the reduced vapour pressurePσ3/ϵ of the order of 0·002. Within that uncertainty, the vapour pressures agree with those obtained previously from perturbation theory. The saturated liquid densities agree within 2 per cent which is consistent with a previous comparison between perturbation theory and experimental results for fluorine. Finally, we note that all simulations were performed with vectorized codes on a CYBER 205.

ISSN:0026-8976    

DOI:10.1080/00268979000100341

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Recently, we have reported diode laser measurements of the halocarbons CCl+and CF+vibrationally excited up to thev= 7 state. In this paper, we present a more detailed analysis of their potential functions as test cases for direct numerical integration of the rovibrational Schrödinger equation. Thef-potential, a new parameterization more suitable for numerical fitting, is derived by mapping the potential function into an analytically simpler representation. The results are compared to the standard semiclassical approaches, which are found to have shortcomings in the precision with which they represent spectroscopic data. These problems can become severe if one desires to determine corrections to the Born-Oppenheimer approximation.

ISSN:0026-8976    

DOI:10.1080/00268979000100351

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A perturbation theory of hard gaussian overlap (HGO) models with electrostatic interactions is proposed. The essentially u-type perturbation method is developed to describe the behaviour of the prolate HGO models with permanent multipole moments. The prolate HGO model with the permanent quadrupole moment is considered in detail and the calculated thermodynamic data are compared with the MC results for the corresponding quadrupolar hard dumbells. A fair agreement is found for the compressibility factor as well as the free and internal energy.

ISSN:0026-8976    

DOI:10.1080/00268979000100361

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

There is an urgent need for accurate anisotropic site-site intermolecular pair potentials for use in realistic simulations of the condensed phases and spectra of van der Waals molecules. However, all attempts to find an analytical form for the repulsion energy have relied on empirical models with many fitted parameters, whose determination requires large quantities of accurate experimental orab initiodata. In this paper, we develop and assess a procedure for estimating the repulsion anisotropy from the monomer wavefunctions which is straightforward, computationally inexpensive and capable of high accuracy. The method is based on the observation that the intermolecular repulsion energy is closely related to the overlap between the unperturbed charge densities of the interacting molecules. The overlap can be treated analytically, leading to an anisotropic site-site functional form. Model pair repulsion potentials with two sites are obtained for (F2)2and (Cl2)2by ignoring the bonding charge density. For (N2)2a novel method of distributing the bonding charge density onto the atoms and the bond centre is used to obtain a three-site model, and this technique is also applied to (F2)2and (Cl2)2. The resulting pair potentials, which have one or two fitted parameters, are found to reproduce anab initiorepulsion surface much more accurately than an isotropic atom-atom potential with two parameters. Since the method uses only the monomer charge densities, it can be extended to larger molecules. The resulting repulsion potential is suitable for incorporation into a complete model of the interaction energy.

ISSN:0026-8976    

DOI:10.1080/00268979000100371

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We report molecular dynamics simulations of the bulk viscosity of a square-well fluid, with a well width equal to half the hard core diameter, at several different densities and temperatures. Our results are compared to analytic treatments by Davis, Rice and Sengers and van Beijeren, Karkheck and Sengers.

ISSN:0026-8976    

DOI:10.1080/00268979000100381

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Potential energy and spectroscopic constants for theX2Σu+ground state of Cl-2, were calculated by configuration-interaction (CI) methods, using large basis sets with polarization and diffuse functions. From these CI wavefunctions, the isotropic (aiso) and dipolar (Adip) components of the hyperfine coupling constant were obtained. The effects of variouss, pbasis sets, polarization and diffuse functions, as well as the influence of reference configurations and configuration selection thresholds were investigated. The best values obtained are 35·31 G foraisoand 29·44 G forAdip, to be compared with experimental values of 37 ± 1 G and 32 ± 1 G, respectively. It is shown that the contributions toaisoof theKandLshells are opposite in sign, differing by about 4 G. Upon vibrational averaging, bothaisoandAdipmove towards smaller values asvincreases. An adiabatic electron affinity of 2·46 eV was obtained for Cl2, and a vertical electron detachment energy of 3·71 eV for Cl-2.

ISSN:0026-8976    

DOI:10.1080/00268979000100391

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An extensive study of the percolation threshold, ρpof three dimensional attractive and repulsive square well fluids is made by Monte Carlo computer simulation. We find that in the hard-core limit an attractive well decreases ρpbelow the high temperature limiting value. In contrast a hard shoulder potential produces the opposite trend. A numerical solution of the PY integral equations employing Hill's concept of bound and unbound pairs gives results qualitatively in agreement with MC. The PY solution overestimates ρpespecially in the hard-core limit and at high temperature. We examine the shapes of all the clusters at the percolation threshold, resolved as a function of the number of particles in a cluster,s. The asphericity parameter,A3, describing the instantaneous shape of the cluster decays slowly from unity, typically only achieving ∼ 0·3 bys∼ 100, close to the estimated universal value of 0·312.

ISSN:0026-8976    

DOI:10.1080/00268979000100401

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor