Coupling the methyl group in acetamide to phonons: a consistent view of tunnelling and lattice dynamics

Inelastic neutron spectra of acetamide-h5 and acetamide-h2 have been measured at different temperatures below 40K and with high energy resolution in the energy range up to 15 meV. A lattice dynamical model on the basis of atom-atom potentials and electric charges obtained from first principles calculations describes the spectra well if soft internal molecular modes are included. The model is used to reproduce the temperature dependence of the tunnelling transition of the methyl group. For this purpose 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 the rotor-phonon coupling are evaluated in second order perturbation theory. The broadening of tunnelling lines comes out correctly when the atom atom potentials taken from literature are scaled down by 20%. To get the shift right the strength of the coupling to the lattice needed to be different for the A type symmetry state from that for the E type state. The coupling with the low frequency phonons appears to be very weak.