A CONSISTENT VIEW OF METHYL ROTATIONAL TUNNELING AND LATTICE-DYNAMICSIN ACETAMIDE

Inelastic neutron spectra of acetamide-h5 have been measured at a samp le temperature T = 15 K with high energy resolution in the energy rang e up to 15 meV. A lattice dynamical model on the basis of atom-atom po tentials and electric charges, obtained from first principle calculati ons, is used. The density of states is split into that of an isolated rotor in a static field and that of the phonon bath. The effects of th e rotor-phonon coupling are evaluated in second-order perturbation the ory. As coupling function the projection of the pure librational onto the pure phonon eigenvectors is used. This procedure describes the spe ctrum in the meV region well if soft internal molecular modes are incl uded and atom-atom potentials taken from literature are scaled down by 20%. The model can also be used to reproduce the temperature dependen ce of the tunnelling transition of the methyl group: the coupling para meter gBAR = 1.9 meV used to describe the density of states reproduces the broadening of tunnelling lines correctly. To get the shift right states of A and E type symmetry are coupled with different strength to the lattice. A coupling to low frequency phonons appears to be neglig ible.