Lattice dynamics and methyl group rotations of 2-butyne

On the basis of the recently determined low-temperature crystal structure, the lattice dynamics of 2-butyne and a single-particle methyl rotational potential were calculated using modified Williams pair potential parameters within the model of semirigid molecules. The agreement of ab initio calculations with neutron spectra of the internal modes shows the validity of this approach. The single-particle librational energy obtained for the rotational potential from the Schrodinger equation of the methyl rotor coincides well with the mean librational band energy. The calculated single-particle tunnelling frequency is only 17% lower than observed in experiment.