Inelastic Neutron Scattering Study of the Torsional and CCC Bend Frequencies in the Solid n-Alkanes, Ethane-Hexane.

High resolution inelastic neutron scattering (INS) spectra of the n‐alkanes (n=2 to 6) have been measured, using the crystal analyzer spectrometer at the intense pulsed neutron source, Argonne National Laboratory, for energy transfer less than 1000 cm−1. In this paper we present these data and extend the valence force field for n‐alkanes of Schachtschneider and Snyder by modifying the torsion potential and the C–C–C bending force constants. The modified force constant set provides a much improved match of the frequencies obtained from normal modes calculations to our experimental data in the energy region below 1000 cm−1 and a minimal perturbation of the frequencies at higher energies. The amplitude of the scattering peaks measured by the crystal analyzer spectrometer is shown to be closely related to the hydrogen density of states. The data also show peaks produced by lattice mode excitations. These data are not analyzed but are compared with data from the literature.