On the structure of liquid phosphorous tribromide (PBr(3)).

We have interpreted the structure factor of liquid PBr(3), as measured by Misawa et al (1990 J. Chem. Phys. 92 5486) on a pulsed neutron source, by means of reverse Monte Carlo (RMC) modelling. We have used the method of fixed neighbour constraints, which accounts for the flexibility of the molecules. From the RMC models, partial pair correlation functions and cosine distributions of intra- and inter-molecular angles were calculated. Comparisons with reference systems, which were generated by hard sphere Monte Carlo simulation, were also carried out. We were able to find unique orientational correlations in liquid PBr(3) in terms of the cosine distribution of [Formula: see text] angles (the Br atom being in the centre), involving one intra-molecular P-Br and one inter-molecular [Formula: see text] nearest-neighbour distance.

Stochastic molecular motions in the nematic, smectic-A, and solid phases of p,p'-di-n-heptyl-azoxybenzene as seen by quasielastic neutron scattering and 13C cross-polarization magic-angle-spinning NMR.

Molecular rotational dynamics in p,p'-di-n-heptyl-azoxybenzene was studied by means of quasielastic neutron scattering (QENS) and 13C cross-polarization magic-angle-spinning (CPMAS) NMR. Fast reorientation of the hydrogen nuclei was observed by QENS in the two liquid crystalline (LC) phases nematic and smectic A, as well as in the crystalline phase. The latter could not be restricted to the -CH3 rotations alone, and a clear indication was found of some other reorientation motions persisting in the crystal. Two Lorentz-type components convoluted with the resolution function gave an excellent fit to the QENS spectra in both LC phases. The narrow (slow) component was attributed to the reorientation of the whole molecule around the long axis. The corresponding characteristic time of approximately 130 ps agreed well with the values obtained in recent dielectric relaxation and 2H NMR studies. The full width at half maximum of the broader (fast) component shows a quadratic Q dependence (Q is the momentum transfer). Hence the corresponding motions could be described by a stretched exponential correlation function and were interpreted as various "crankshaft-type" motions within the alkyl tails. The 13C CPMAS experiments fully corroborated the QENS results, sometimes considered ambiguous in complex systems.

The high resolution inelastic neutron scattering spectrum of ammonium fluoride.

The measured high resolution (deltaE/E approximately 2-3%) incoherent inelastic neutron scattering spectrum of ammonium fluoride is presented and discussed with reference to the available optical spectra. In addition, a full set of dispersion curves have been obtained from a new ab initio lattice dynamics calculation and these have been used to produce a rigorous interpretation of the spectrum. The librational modes of the ammonium ion occur at 560 cm(-1) and the anharmonicity in these modes is estimated to be 4%, about half that observed in the other ammonium halides. The reduction in anharmonicity is attributed to stronger hydrogen bonding and a deeper potential well. The calculations agree well with the observed spectrum apart from the librational modes which are shifted upwards by around 40 cm(-1) from the measured values. Dispersion and LO-TO splitting are important in this system with modes changing frequency by up to 135 cm(-1). The nature of the calculated LO-TO splitting in the optic mode region is indicative of a pseudo-cubic system confirming that the site symmetry of the ammonium ion is very close to T(d). Because of LO-TO splitting the ammonium ion asymmetric stretch, nu3, has components calculated to be at higher frequencies than those of the symmetric stretch, nu1, which contradicts the assignment scheme produced from optical data.

Analytical solution for the Feynman ratchet.

A search for an analytical, closed form solution of the Fokker-Planck equation with periodic, asymmetric potentials (ratchets) is presented. It is found that logarithmic-type potential functions (related to "entropic" ratchets) allow for an approximate solution within a certain range of parameters. An expression for the net current is calculated and it is shown that the efficiency of the rocked entropic ratchet is always low.