摘要:

We derive the rules for collisions between hard spheres in arbitrary dimensions in the isobaric-isoenthalpic ensemble. The static and time-dependent properties of the one-dimensional hard rod system are investigated in detail, both theoretically and numerically.

ISSN:0026-8976    

DOI:10.1080/00268978600102301

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Smoluchowski's rotational diffusion equation for molecules in the liquid state is solved by statistical perturbation calculus and applied to describe the time-evolution of the Kerr effect induced by a reorienting pulse field. General formulae are proposed for rectangular and cosine pulses as well as ones composed of the two permitting discussion of the rise and decay transients of birefringence on switching on and removal of the pulse.

ISSN:0026-8976    

DOI:10.1080/00268978600102311

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

We have studied the surface behaviour of the Lebwohl-Lasher model nematogen using the Monte Carlo technique of computer simulation. Two states of the nematogen have been investigated, one with normal periodic boundary conditions and the other with boundary conditions employed for just two dimensions thus forming two free surfaces orthogonal to the third dimension. We have determined the variation of the internal energy and the second rank orientational order parameter, associated with individual layers, with their distance from the surface, as a function of the temperature. The order in the surface is found to be less than in the bulk nematogen although this difference is far less for the isotropic phase than for the nematic. This suppression of the surface orientational order is in accord with the surface tension simulated for the nematogen. Thus the surface tension decreases rapidly with increasing temperature in the nematic mesophase but then is essentially independent of temperature in the isotropic phase. Direct evaluation of the surface tension suggests that, to within the statistical error of the simulation, this is continuous at the nematic-isotropic transition. However, we have been able to show that the surface tension of the nematic phase is slightly greater than for the isotropic phase from the small reduction in the transition temperature produced by the creation of the free surfaces. We have also developed a molecular field theory for the surface properties of the Lebwohl-Lasher nematogen and this has been tested using the results of our Monte Carlo simulation. In particular the theory greatly overestimates the reduction in the surface tension at the transition from the nematic to the isotropic phase. However other properties, such as the variation of the order parameter with the separation from the surface and the temperature dependence of the surface tension, are predicted in better agreement with the results of the simulation.

ISSN:0026-8976    

DOI:10.1080/00268978600102321

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Proton second moment and spin-lattice relaxation times (T1andT1ρ) have been studied in solid 2-methyl-2-propanol in the temperature interval 77 K to the melting point. The data at low temperatures are satisfactorily explained in terms of simultaneous methyl and t-butyl motions. The activation energies of these motions are of 22·3 and 13·2 kJ mol-1, respectively. At high temperatures the relaxation, particularlyT1ρ, is influenced by translational diffusion. The energy barrier to this process is estimated roughly to be of about 137 kJ mol-1. The results confirm the existence of the phase transition at around 287 K and indicate that it may originate from the coupling between internal motions.

ISSN:0026-8976    

DOI:10.1080/00268978600102331

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Second-order calculations of interatomic interactions in the van der Waals region are presented for the simple model diatomics H2+(2Σu+), H2(3Σu+) and He2(1Σg+). A non-orthogonal Epstein-Nesbet expansion starting from the Hartree-Fock ψ0of the individual atoms and including pseudostates accounting for the first three non-expanded multipoles with orbital exponents fixed to their asymptotic values is seen to give in this region reasonably accurate results for the potential, which is the resultant of repulsive first-order and attractive second-order induction and dispersion energies including exchange.

ISSN:0026-8976    

DOI:10.1080/00268978600102341

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Differential reaction cross-sections for the K +RI →KI+R(R= CH3, C2H5, C3H7) systems have been measured as a function of the collision energy by using our molecular beam apparatus. The analysis of the centre-of-mass angular and recoil velocity distribution of the products indicated: (a) a backward peak character corresponding to a direct, rebound mechanism (b) that the average translational energy of the products <E'T> increases approximately linearly with increasing collision energy,ET, as follows <E′T>/kJ mol-1=a<ET>/kJ mol-1+bwhere (a, b) are (0·65, 66·5), (0·56, 58·4) and (0·28, 30·3) for the methyl, ethyl and propyl iodide reactions respectively. Whereas in the methyl and ethyl reactions the fraction of the available energy that appears in translation,f′T, is about 0·65–0·55 in the propyl case it falls toca.0·3. A comparison with photofragmentation studies for the alkyl iodides,RI, has shown similar dynamics involving a ‘quasidiatomic’ C-I excitation followed by recoil of a ‘soft’ radical. An information theoretical analysis of the reaction products' recoil velocity distribution revealed that (direct) dynamics involving the rupture of the same C-I bond can recover the experimental differential reaction cross-sections providing that the statistical contribution, associated with the degrees of freedom of the different radical groups, is properly increased. In particular the enhancement of the energy disposal for the propyl reaction could be associated with the presence of the symmetric C-C-C bending motion.

ISSN:0026-8976    

DOI:10.1080/00268978600102351

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The rotational motion of molecules about the three space axes in the liquid state can be described by the rotational diffusion tensor. Until recently most investigations concerning rotational motions of molecules in liquids employing dipolar spin-lattice relaxation data have proceeded on the assumption that the orientation of the rotational diffusion tensor principal axis system coincides with that of the intertial tensor. But this is not necessarily the case for liquids which consist of molecules rotating as asymmetrictop rotors with less than two mutually perpendicular mirror planes. Usually it is assumed that, in hydrocarbons, a rotation of the two principal axis systems with respect to each other does not occur because of their weak intermolecular interactions. In order to examine this statement the rotational behaviour of the molecules of the hydrocarbon 1,2,3,4-tetrahydro-5,6-dimethyl-1,4-methanonaphthalene(1)was investigated using13C dipolar spin-lattice relaxation time measurements. The orientation of the rotational diffusion principal axis system was determined by successive variation of the eulerian angles between the two principal axis systems, and, two favoured orientations were obtained which were quite different from the orientation of the inertial tensor principal axis system. For both orientations the rotational diffusion constants and the methyl group jumping rates were evaluatedviaan extended Woessner-formalism developed by Bluhm (1982).

ISSN:0026-8976    

DOI:10.1080/00268978600102361

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

A perturbation theory for the high frequency elastic moduli of colloidal dispersions is developed using an inverse power potential reference fluid. When supplemented by a random phase approximation for the perturbation potential, the theory can be used to determine rheological properties of disordered colloidal dispersions from observed structure factors. The theory is applied to measurements of the structure factor for an aqueous dispersion of polystyrene latex spheres at the order-disorder transition. The calculated properties are found to be in good agreement with independent experimental measurements.

ISSN:0026-8976    

DOI:10.1080/00268978600102371

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

We present molecular dynamics simulation of a lipid bilayer of 2 × 10 freely jointed chain molecules of 16 beads each. All beads interact with each other (via a Lennard-Jones potential) and with an aqueous medium. Terminal beads are bound near the bilayer surfaces by a harmonic potential. An algorithm for integrating the equations of motion of a molecular system in the presence of geometrical constraints, such as fixed bond lengths, is described. All results are compared with those obtained for an isotropic polymer liquid with the same Lennard-Jones interaction and density as for the bilayer. Equilibrium properties of the chains such as end-to-end distance, radius of gyration, molecular order parameters and pair correlation functions are computed. Dynamic properties considered include the time autocorrelation functions of various variables, the translational motion of chain beads and the space-time van Hove correlation functions. It is found that compared with the polymer liquid, the molecular motions in the anisotrpic bilayer and its structure are quite different. Such differences arise mainly from the strong correlations existing in the bilayer.

ISSN:0026-8976    

DOI:10.1080/00268978600102381

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The formalism developed by Meeron and Siegert is modified in such a way that the logarithm of the background correlation function is expressed in terms of the chemical potential of hard spheres and that of a fused hardsphere body in a mixture with hard spheres. The recently proposed equation of state is employed to evaluate these chemical potentials. The background correlation functions on the interval (0, √2) can be easily evaluated with high accuracy for all densities in one- and multicomponent systems. The proposed method allows us to determine triplet and higher-order correlation functions.

ISSN:0026-8976    

DOI:10.1080/00268978600102391

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor