MEASURED AND CALCULATED ROTATIONAL TUNNELING DYNAMICS IN METHYL ACETATE

A low-field NMR and inelastic neutron scattering investigation of tunn elling methyl dynamics in methyl acetate has been performed. Tunnel fr equencies of 1.3 mu eV and 0.31 neV have been measured for the two che mically distinct methyl groups in the molecule. From the known crystal structure at 145 K, the observed quantum methyl dynamics can be repro duced from a time-independent, molecular mechanics calculation based o n ab initio techniques and empirical atom-atom potentials. Although no spectral evidence of coupled methyl dynamics exists, the delocalisati on and reorientation of the methyl groups at low temperature must be i ncluded in the calculation in view of the strong coupling potentials. This is achieved with a quantum mechanical mean-field approach. (C) 19 98 Elsevier Science B.V.