摘要:

A lattice dynamical calculation for the known crystal structure of orthoterphenyl has been carried out under the rigid molecule approximation. The optimized parameters for the intermolecular potential reproduce adequately both the equilibrium crystal structure as well as the experimental heat capacity up to 35 K. The values for the mean-square-displacement as calculated from the computed vibrational density of states show that for temperatures well above 50 K most of the motions contributing to the atomic displacements as measured by neutron quasielastic scattering arise from the thermally excited internal molecular modes. The implications for the recent tests of kinetic theory predictions regarding the liquid-glass transitions using this material are briefly discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300103021

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The symmetry of coefficients in the expansion of a given molecular property are shown to be obtainable through well defined tensor methods. The analytical expansions required of various vibrational and rotational tensors are given up to the fourth degree in dynamical variables. To illustrate the method, some rovibrational properties of spherical tops are considered. The symmetry of the associated molecular constants is expressed, simultaneously, for XY4and XY6molecules, by algebraic relations involving the independent constants and the coupling symbols of the molecular symmetry group. To allow comparisons, some examples are given for which the symmetry of the constants has been published already in the form of tables; the more complicated cases of the third order ℒ tensor elements and of the quintic potential derivatives are also considered.

ISSN:0026-8976    

DOI:10.1080/00268979300103031

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The rotational spectrum of 1,3-benzodioxole has been investigated in the frequency range 30–40 GHz. Six vibrational satellites have been observed and assigned to excited states of the puckering and twisting modes of the five-membered ring moiety. On the basis of the observed zigzag behaviour of the rotational constants upon vibrational excitation, the puckering has been shown to be controlled by a double-minimum potential. Similar changes upon vibrational excitation have been observed for the centrifugal distortion constant ΔJKand for the dipole moment component μa. These properties have been interpreted by a one-dimensional flexible model for the ring-puckering motion. A two-dimensional model that accounts for both the puckering and the butterfly motion of the two rings contained in the molecule was used to analyse the available far-infrared data and has led to the assignment of 19 observed absorption lines. This analysis has yielded a ring-puckering barrier of 126 cm-1and an equilibrium structure in which the oxygen nuclei and the methylenic carbon nucleus are on different sides of the plane that was assumed to be formed by the benzene ring.

ISSN:0026-8976    

DOI:10.1080/00268979300103041

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Solid state13C NMR spectroscopy has been applied to investigate the time scale and the geometry of rotational motions near the glass transition of the low-molar-mass van der Waals glass former 1,3,5-tri-α-naphthylbenzene (Tg= 342 K). Two-dimensional13C exchange spectra were taken between 347 and 372 K. With the principal values of the13C chemical shift tensors determined from13C CP/MAS spectra and static one-dimensional13C spectra, the two-dimensional spectra are interpreted according to the isotropic rotational diffusional model to yield mean correlation times τ0in the range 10 ms ˇ- τ0ˇ- 50 s, and a log-Gaussian distribution of correlation times with a width of 1·5 decades. The mean correlation times agree with scaled viscosity data and with results from photon correlation spectroscopy.

ISSN:0026-8976    

DOI:10.1080/00268979300103051

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Electrochemical double-layer emersion is a powerful tool for probing molecular and electronic structural properties of the electrode-electrolyte interfacial region. It also poses some interesting questions as to the potential distribution in the emersed layer and to potential monitoring outside the emersed layer.

ISSN:0026-8976    

DOI:10.1080/00268979300103061

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have evaluated the bulk and surface elastic constants of a nematic liquid crystal composed of Gay-Berne particles, using a generalization of earlier theories by Poniewierski and Stecki, and Lipkin, Rice and Mohanty. The ratios between surface and bulk elastic constants have been plotted as a function of temperature and at nematic-isotropic coexistence. In particular, the sign of theK13(splay-bend) constant has been shown to depend sensitively on details of the intermolecular potential. Results have been contrasted with those obtained from a different theory.

ISSN:0026-8976    

DOI:10.1080/00268979300103071

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The problem of collective molecular coordinates is examined within the Hamiltonian formalism by enlarging the number of degrees of freedom and transforming the model to a gauge theory (a theory where the solutions to the equations of motion include arbitrary functions of time). It turns out that in the case of a nonlinear molecule and within the harmonic approximation the most appropriate gauge fixing conditions are the Eckart conditions; thus the Wilson-Howard Hamiltonian is derived. The case of a linear molecule is examined as a second application, and it is found that there are two reasonable gauge choices. The question of the linear molecule as the limiting case of a nonlinear one is also elucidated.

ISSN:0026-8976    

DOI:10.1080/00268979300103081

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In a polarized neutron diffraction experiment on the magnetically highly anisotropic ammonium cobalt Tutton salt (ND4)2Co(SO4)2·6D2O, 70 and 22 flipping ratios were measured in the hk0 and h01 zones respectively. These, and the underlying magnetization distribution around Co, more closely resemble those predicted by a model with a cubic crystal field with CoO6axes, than those predicted from a generally orientated ellipsoidal crystal field, even though the latter provides a good fit to ESR and magnetic susceptibility data. However, all the real space magnetization distributions are substantially isotropic. For an examination of covalence as the experimental objective, these results indicate that a suitably scaled spin-only model for the magnetization distribution on the metal may suffice, even in highly anisotropic situations where a large orbital moment is present, such as the present case.

ISSN:0026-8976    

DOI:10.1080/00268979300103091

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A recently developed non-equilibrium molecular dynamics algorithm for heat conduction is used to compute the thermal conductivity, thermal diffusion factor, and heat of transfer in binary Lennard-Jones mixtures. An internal energy flux is established with local source and sink terms for kinetic energy.

ISSN:0026-8976    

DOI:10.1080/00268979300103101

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Intermolecular pair potentials of methane and of methane + nitrogen have been calculated by quantum chemicalab initiomethods. The repulsive and electrostatic parts were determined pointwise for various distances and orientations of the dimers (supermolecule approach) by self-consistent field (SCF) calculations including the counterpoise correction. Gaussian basis functions of triple-zeta quality plus one set of polarization functions on all atoms were used. The dispersion energy, which cannot be calculated at the SCF level, has been added by a semi-empirical estimate. These potentials and the pair potential for nitrogen (of similar quality, taken from the literature) were fitted to analytical functions and used forNVTMonte Carlo simulations of thermodynamic properties of the fluids and their mixture over a wide temperature/density area. Comparison with measurements and with Monte Carlo results from the literature (pressure, internal energy, radial distribution function) obtained with other pair potentials indicates the quality of the present calculations.

ISSN:0026-8976    

DOI:10.1080/00268979300103111

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor