Lattice dynamics and methyl rotational excitations of 2-butyne

Abstract

On the basis of the recently determined low temperature crystal structure the lattice dynamics of 2-butyne and a single particle methyl rotational potential are calculated using pair potential parameters given by Williams in 1974 within the model of semirigid molecules. In the regime of lattice modes the existence of four methyl librational bands with significant dispersion can explain the measured density of states. The single particle librational energy obtained for the rotational potential from the Schrödinger equation of the methyl rotor coincides well with the mean librational band energy. The calculated single particle tunneling frequency is only 17% lower than observed in experiment. Similarly well the activation energy is reproduced. Although the pair potential parameters developed for aromatic compounds need some scaling they are useful for materials with triple bond carbons.