Vibrational spectra of crystalline n-paraffins : II. Intermolecular effects

Abstract

Intermolecular effects in the infrared spectra of crystalline n-paraffins, n-C 20H 42 through n-C 30H 62 are reported. Absorption bands in the spectra of the triclinic form are singlets. Extensive doubling of bands occurs for the mono clinic and orthorhombic structures in accordance with selection rules based on the factor group symmetry. In particular, the methylene rocking mode bands of the orthorhombic structure display a striking pattern of splitting. Frequency separations of the components can be expressed by a simple function of a single parameter, kπ (m + 1) , where m is the number of methylene groups of the n-paraffin and k is an integer which characterizes a particular intramolecular mode. From these data several intermolecular force constants have been evaluated. In relating these force constants to specific methylene interactions, methylenes in adjacent planes were found to interact more strongly than those in the same plane. By extending Stein's theory that the splitting arises from short range repulsive forces, to include additional interactions, the force constants are explained satisfactorily. However, interactions which involve only dipole-dipole forces are found to be inadequate.