Methyl group tunnelling and coupling to internal vibrations

A coupling between the librations and internal vibrations is suggested to decrease the librational tunnelling frequency nu t of the methyl group at low temperatures. The coupling originates from a low symmetry of the lattice potential at the site of the group. The experimentally observed nu t at liquid helium temperatures is shown to be related to the ground and excited librational states. The initial temperature dependence of the coupling is determined by the methyl group distortion. A distortion becoming smaller when the temperature is raised is shown to make nu t larger.