Knowledge Bank

The torsional and sketal bending fundamentals in solid n-pentane, n-hexane, n-heptane, and n-octane are measured by the method of cold neutron inelastic scattering. For the series of n-paraffins studied, the maximum frequencies of the skeletal torsion and skeletal bending modes approach $190cm-1$ and $500cm-1$ respectively, in agreement with previous measurements on polyethylene. The average value of the two end-methyl torsions is found to be $240cm-1$. A normal coordinate analysis, performed using only intra-molecular valence force fields for each molecule, is used as a guide in spectral assignments. These results are compared to the predictions of the infinite chain model. Chain-end effects appear to be of little significance for these low frequency modes, even in chain lengths as short as 7 or 8 carbons.