摘要:

Several methods currently used to calculate isotropic dispersion energy coefficients,C2n, are investigated: (i) a version of the single excitation energy Unsöld scheme, (ii) a ‘bond’ oscillator model (which employs Frost model wavefunctions), and the generalizedab initio(iii) Unsöld and (iv) Kirkwood schemes. The large numerical discrepancies, which often occur in the results forβ=C8/C6and γ=C10/C6obtained by the various methods, are ascribed to difficulties with (i) and (ii). Corrected versions of (i) and (ii) are discussed and they yield larger results for β and γ which, especially in the case of (i), are in much better agreement with those of theab initioschemes (iii) and (iv) which show the expected marked increase in β and γ as the size of the interacting species increases. Results from theab initioschemes, using SCF wavefunctions, are presented forC6,C8andC10for interactions arising from the molecules BH3, CH4, NH3, H2O, HF, CO, CO2and N2O. The numerical values for theC2nare still subject to rather large uncertainties although for some interactions the results are apparently (fortuitously) very good. The results for β and γ are much more reliable than the values of the individualC2nand hence can provide a reasonably reliable description of isotropic long range interactions when combined with accurate values forC6.

ISSN:0026-8976    

DOI:10.1080/00268978000102751

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

A general formalism is presented to calculate the neutron incoherent scattering law for diffusion in a potential of spherical symmetry and applied to the problem of diffusion of a particle in the interior of a sphere with an impermeable surface. The shape of the spectra and their behaviour versus the momentum transfer predicted by this model is discussed and compared to those of simpler models. This model should satisfactorily describe molecular diffusion in restricted geometries in three dimensions, e.g. water molecules around hydrophilic centres in solids.

ISSN:0026-8976    

DOI:10.1080/00268978000102761

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The second dielectric virial coefficients of CH4and CF4are calculated by only taking into account the dipole-induced dipole contribution for the distortion part and the octupolar and hexadecapolar fields for the orientation part. The resulting values are consistent with experiment. The contribution of the electronic correlation and exchange effects to the collision induced polarization is discussed.

ISSN:0026-8976    

DOI:10.1080/00268978000102771

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The aim of this paper is to present some applications of a method indicated by Sears in order to correct for multiple scattering. The calculations were performed in the particular case of slow neutron incoherent quasielastic scattering from organic plastic crystals. First, an exact calculation (up to second scattering) is compared with the results of a Monte Carlo simulation technique. Then, an approximation is developed on the basis of a rotational jump model which allows a further analytical treatment. The multiple scattering is expressed in terms of generalized structure factors (which can be regarded as self convolutions of first order structure factors taking into account the instrumental geometry) and lorentzian functions the widths of which are linear combinations of the jump rates. Three examples are given. Two of them correspond to powder samples while in the third we are concerned with the case of a single crystalline slab. In every case, this approximation is shown to be a good approach to the multiple scattering evaluation, its main advantage being the possibility of applying it without any preliminary knowledge of the correlation times for rotational jumps.

ISSN:0026-8976    

DOI:10.1080/00268978000102781

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

For different models of molecular motions, correlation times of cubic reorientations are calculated from the intramolecular1H spin lattice relaxation time determined by studying powdered solid solutions of C10H16and C10D16at room temperature. The effect of isotropic librations on intramolecular spin-lattice relaxation and second moment has been calculated for single crystal and powder samples. The best agreement with the correlation time values calculated from powder neutron scattering experiments is obtained for a model where the molecules reorient by 90° jumps about the crystallographic C4axes: τc4-1=(5·3 ± 0·5)1012exp [-(1395 ± 20)/T[°K]] s-1. This conclusion is in agreement with a detailed analysis of incoherent neutron scattering on single crystal of adamantane (preceding paper).

ISSN:0026-8976    

DOI:10.1080/00268978000102791

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Using the incoherent quasielastic neutron scattering technique, we have studied the molecular reorientations of 1-cyanoadamantane C10H15CN in its plastic phase. The measurements were done at eight temperatures from 256 K to 423 K using a powder sample. Multiple scattering effects were taken into account using a semianalytical method previously described in another paper. It is shown that molecular motions consist in reorientational jumps between twelve equilibrium positions equally spaced by 30° around the [100] directions of the lattice.

ISSN:0026-8976    

DOI:10.1080/00268978000102801

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

A perturbation theory for predicting the thermodynamic properties and structure of a molecular liquid is described. The theory is applied to a two-centre Lennard-Jones fluid and the properties of this fluid are calculated from the properties of a hard dumbell fluid. The liquid structure of the dumbell reference system is calculated using the RISM equations. The free energy of the reference system is calculated by scaled particle theory and by Monte Carlo simulation. The non-spherical hard-core system is softened before the anisotropic perturbation is added.

ISSN:0026-8976    

DOI:10.1080/00268978000102811

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Zeeman and variable temperature experiments on the lowest (optically excited) triplet state of palladium porphin (PdP) in single crystals ofn-octane (n-C8) andn-decane (n-C10) performed at low temperatures are reported. The orbital degeneracy of the3Eustate of the free molecule appears to be lifted in the crystal and the splitting between the two orbital components |x> and |y> amounts to 57 cm-1forn-C8and 41 cm-1forn-C10. The triplet state parameters for PdP inn-C8areD= - 23 ± 5 GHz,g⊥= 2·00 ± 0·02,g‖= 1·68 ± 0·04, and for PdP inn-C10they areD= -42 ± 5 GHz,g⊥= 2·00 ± 0·01,g‖= 1·51 ± 0·03. The zero field splitting andg-tensor anisotropy are dominated by contributions from the spin-orbit coupling which connects the τx, τylevels of the3|x> with those of the3|y> state by a matrix element which has a magnitude ofZ′ = 14·6 ± 0·3 cm-1. The orbital angular momentum in units of ħ in the3Eustate amounts to Λ′ = 1·4 ± 0·1. In accordance with theoryZ′ and Λ′ appear to be independent of the host. The directions of the canonical axes in the lowest triplet state of PdP in theA-site ofn-octane have been determined with respect to the crystallographic axis system.

ISSN:0026-8976    

DOI:10.1080/00268978000102821

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

A single configuration (minimal basis set) diatomics-in-molecules (DIM) approach is used to calculated the ground and low-lying excited electronic states of the H3molecule, corresponding to a C2vsymmetry. It is shown that this version of the DIM method is consistent with the neglect-of-overlap approximation. The required diatomic potential energy curves are constructed on the basis ofab initiodata and are interpolated with cubic spline functions. The results obtained are discussed and compared withab initiodata.

ISSN:0026-8976    

DOI:10.1080/00268978000102831

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The effects of electronic angular momentum in triatomic linear molecules are considered. An effective vibronic hamiltonian is derived and second order energy level expressions are obtained for the bending levels in an electronic Π state. The formulae allow for the anharmonicity of the bending potentials and for the variation of the expectation value <Lz> with bond angle; effects of electron spins are also included. The vibronic levels predicted by the analytic expressions are compared with those calculated using a full matrix treatment of the orbital angular momentum; it is shown that they are far more accurate than the levels predicted by the formulae currently available in the literature. The relationship between the anharmonic corrections and the deviation of <Lz> from unity is discussed in terms of an electrostatic interaction between linear molecule states of different symmetry.

ISSN:0026-8976    

DOI:10.1080/00268978000102841

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor