Motions of Molecules in Condensed Systems. X. The Infrared Spectrum and Structure of a Single Crystal of Cyclopropane

Abstract

The absorption of polarized infrared radiation by thin sections of single crystals of cyclopropane, grown in a specially designed apparatus, has been recorded in the frequency range between 700 and 5000 cm−1, and compared with spectra, similarly recorded, of liquid and vapor. The assignment of molecular modes is completed: The ν5 fundamental, not previously measured, is located in the crystal at 1078 cm−1, while ν4 and ν13 appear at 1133 and 1194 cm−1, respectively, confirming previously uncertain assignments. The frequencies of the primary lattice modes of the molecule in the crystal are inferred from combinations. Site group and factor group splittings are seen to be of the same magnitude (≈10 cm−1). The cyclopropane crystal is found to be orthorhombic (C2v or D2h), with the molecules located at sites of Cs symmetry. An arrangement of the molecules, described by space group Pnm (C2v7), which gives the most efficient packing, is proposed as the actual crystal structure.