Inelastic neutron scattering spectra of the transverse acoustic modes of the normal alkanes

The inelastic neutron scattering spectra of the n-alkanes, obtained at ca. 25 K, have been examined in the frequency region below 260 cm^-1. Assignments of the transverse acoustic modes (TAMs) of pentane to dodecane out to wavevectors, κ, of about 0.6 are presented. These emerge from a general scheme, which is broadly applicable to all alkanes but inconsistent with single molecule ab initio calculations (because of the mode mixing introduced by the presence of external forces). The external vibrational spectra of the n-alkanes were reported for the first time and shown to have an n-even, n-odd, dependence, in agreement with the variations reported for their crystal structures. However, the demonstration of a similar n-even, n-odd, alternation in the methyl torsion frequencies must be understood in terms of the internal, and not the external, forces.