摘要:

A temperature perturbation method for a direct Monte Carlo simulation of the residual free energy is extended to the isothermal–isobaric ensemble. Unlike many other free energy or phase equilibrium methods, particle insertion is avoided. At the same time, the usual Boltzmann sampling is maintained. The results show excellent agreement with equations of state and thermodynamic integration for fluids of hard dumbbells and linear chains of tangent hard spheres. The results cover states at which insertion techniques fail.

ISSN:0026-8976    

DOI:10.1080/002689796172968

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The relation between molecular helicity and the handedness of cholesteric phases was studied investigating a model system of chiral atropisomers using Monte Carlo simulations in theNVTensemble. The system under investigation was given by N = 256 rigid, chiral Gay–Berne atropisomers of point symmetry group D2. The molecular chirality of the atropisomers was introduced by joining two Gay–Berne particles through a bond with a suitable fixed dihedral angle. Starting from an isotropic configuration, cholesteric phases were obtained on equilibrating the systems whereby left-handed cholesterics were formed by atropisomers with M helicity and right-handed cholesterics by atropisomers with P helicity. A11 phases have been characterized by visual representations of selected configurations, correlation functions, and sets of four local order parameters.

ISSN:0026-8976    

DOI:10.1080/002689796172977

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We report simulation results of chemical potentials on argon–krypton and nitrogen–ethane mixtures calculated by three methods, namely, the Widom test particle method, the difference method, and the ‘fA–fB’ method. The results showed that the ‘fA–fB’ method is best among the three methods for the calculation of chemical potentials at high densities, and capable of dealing with the liquid–liquid phase equilibria of mixtures.

ISSN:0026-8976    

DOI:10.1080/002689796172986

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The microwave transitions for the (15NO)2dimer were reinvestigated following a report by McKellaret al. (1995, Molecular Physics,86, 273) of a discrepancy between a previously measured microwave transition frequency and the corresponding frequency calculated from their molecular constants. The present microwave measurements confirm the previous observation of a rotational transition at 11159•297(4) MHz, but this transition is apparently not due to thecisN—N bonded NO dimer, since a mixture of15NO in argon gas is required for observation of the 11159•297 MHz transition. A new doublet was located at 11128•434(4) MHz and this transition is now assigned to the 303← 212transition of the15N—15NO dimer, in agreement with predictions from the rotational constants of McKellaret al. This transition can be observed with either argon or neon as the expansion gas. New structural parameters obtained from the microwave data are in excellent agreement with those based on the FTIR data.

ISSN:0026-8976    

DOI:10.1080/002689796172995

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We have investigated the localchain conformation in amorphous bisphenol-A polycarbonate at the carbonate–phenylene fragment by a solid-state nuclear magnetic resonance study. Chemical-shielding anisotropy (CSA) powder patterns have been analysed, and their sensitivity to local conformation is discussed. CSA–dipole tensor correlation experiments on isotopically enriched samples were analysed in terms of dihedral-angle distributions. The comparison with atomistic simulations allowed for a critical test of the glass model and provided information for the improvement and refinement of the model.

ISSN:0026-8976    

DOI:10.1080/002689796173002

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In an effort to elucidate the nature of general reactions between carbynes and hydrocarbons, the reactants, products and transition state for the reaction of methylidyne (CH) with methane (CH4) have been characterized usingab-initioquantum-mechanical methods. Both the self-consistent field (SCF) method and the method of configuration interaction of including single and double excitations (CISD) were employed initially. The largest basis used was a triple-ζ plus double-polarization plus f-functions basis (TZ2P+f). All levels of theory predict that this reaction proceeds through a transition state of Cs symmetry. With the TZ2P+f basis, the SCF method predicts an activation energy of 25•8 kcal mo1-1while CISD predicts 9•5 kcal mo1-1. Coupled cluster methods including all single and double excitations (CCSD) applied to the optimized TZ2P+f CISD geometries yield an activation energy of 5•8 kcal mo1-1. When connected triple excitations are added perturbatively to the CCSD energy (CCSD(T)), the activation energy is further reduced to 4•3 kcal mo1-1. The estimated exact activation energy is 3•5±0•5 kcal mo1-1.

ISSN:0026-8976    

DOI:10.1080/002689796173011

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Exact results are obtained for isolated, freely jointed, square-well trimers bonded with an infinitely deep square-well potential. Bond flexibility is found to have small but detectable effects on structural and thermodynamic properties. A larger chain size, a higher heat capacity, and a sharper collapse transition with a lower transition temperature are observed for a flexibly bonded chain, relative to an infinitely stiff trimer.

ISSN:0026-8976    

DOI:10.1080/002689796173020

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The polarization labelling spectroscopy technique is applied to study the A ¹Σu+– X ¹Σg+band system of sodium dimer. Six visible lines of an argon ion laser are used to label rovibronic levels in the ground state of Na2. Rotationally resolved polarization spectra are observed in the range 14200–17600 cm-1. A set of Dunham coefficients is deduced to fit all unperturbed levels of the A ¹Σu+state with 0 ⩽ ν ⩽ 43 and 12 ⩽ J ⩽ 126.

ISSN:0026-8976    

DOI:10.1080/002689796173039

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In this work we simulate the permeation of Lennard-Jones gases across a zeolite model membrane. Using a newly developed dual control volume grand canonical molecular dynamics technique, we create spatial variation in the chemical potential in a dynamical system and hence an accurate simulation of steady-state pressure-driven diffusion. The molecular sieving nature of microporous zeolites is discussed, and the results from the simulation are compared very favourably with recent experimentalresults of He, H2and CH4permeation through ZSM-5 polycrystalline membranes. A massively parallel algorithm is utilized to give a quick and insightful study of this and other microporous materials for use as membranes.

ISSN:0026-8976    

DOI:10.1080/002689796173048

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A recently developed method for the simulation of chemical potentials of chain molecules (EVALENCH) is applied here to obtain the chemical potential, the mean square end-to-end distance (Rn2) and the mean square radius of gyration (Rg2) of dilute chains in different athermal media. The environments considered in this work are a frozen network structure, a deformable network matrix and a monomeric solvent at various densities. The properties of all chain lengths smaller than a preset maximum are calculated in a single simulation. A novel method is also presented for locating and computing the fraction of sampling space available to append one segment of an existing chain. This method enhances the range of densities where simulations of chemical potential are feasible. Simulated chemical potentials are compared with the predictions of two theories; good agreement is found in both cases. We find that Rn2and Rg2are reduced as the density of the medium is increased (network or solvent), while they are increased when the network is frozen and as the monomeric solvent size is made larger than that of the chain sites. At the conditions studied here, no direct evidence of chain collapse is observed.

ISSN:0026-8976    

DOI:10.1080/002689796173057

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor