Rotational dynamics of methyl groups in durene: A crystallographic, spectroscopic, and molecular mechanics investigation

Neutron powder diffraction measurements of perdeutero durene in the tempera ture range from 1.5 K to 290 K have been performed. The lowest temperature structure is the starting point for calculations of the methyl group tunnel ing and librational dynamics. Ab initio methods and atom-atom potentials ar e used to determine rotational single particle and coupling potentials. Tun neling splittings and librational bands are calculated by numerical solutio n of Schrodinger's equation for a system of many coupled methyl groups. Hig h-resolution inelastic neutron scattering measurements of methyl tunneling and molecular vibrations have been repeated, the tunneling results resolvin g an inconsistency with earlier NMR work. Quantum molecular dynamics provid e a stringent test of the numerical methods and the data are ultimately wel l reproduced. These results are also discussed in the context of optical me asurements of dye molecules in a host lattice of durene, (C) 1999 American Institute of Physics. [S0021-9606(98)01048-4].