Low-frequency molecular vibrations in solid n -paraffins by neutron inelastic scattering: n -Pentane, n -hexane, n -heptane, and n -octane

Abstract

The torsional and skeletal bending fundamentals in solid n -pentane, n -hexane, n -heptane, and n -octane are measured by the method of cold neutron inelastic scattering. For this series of n -paraffins, the maximum frequencies of the skeletal torsion and skeletal bending modes approach 190 and 500 cm−1, respectively, in agreement with previous measurements on polyethylene. The average frequency of the two end-methyl torsions is found to be 240 cm−1. A normal coordinate analysis, performed using only intramolecular valence force fields for each molecule, is used as a guide in spectral assignments. These results are compared to theoretical dispersion curves of polyethylene, with and without intermolecular forces. Chain-end effects appear to be of little significance for these low-frequency modes, even in chain lengths as short as seven or eight carbons.