Vibrations of the n-alkanes and the nonrigid molecular symmetry group

Abstract

The nonrigid molecular symmetry groups of all of the n-alkanes (except methane and ethane) are shown to be isomorphic to one another. The methyl group vibrations span two-dimensional (as well as one-dimensional) irreducible representations within this group. The energies of the methyl stretching modes are well described by this group while the rocking modes require a point group description. The bending modes are intermediate. The selection rules are best described by the point group for all modes. The expanded group does not provide any new information for the methylene vibrations.