Motions of Molecules in Condensed Systems. XII. Infrared Spectrum and Structure of a Single Crystal of Allene

The absorption of polarized infrared radiation by thin single crystals of allene at 95°K, and again at 125°K, has been recorded in the frequency range between 700 and 3500 cm—1. Spectra of liquid and vapor phases were also recorded under equivalent conditions. All absorption bands have been measured and assigned. The frequencies of the primary lattice modes of the molecule in the crystal are inferred from combinations, and show a strong dependence on temperature. The allene crystal is found spectroscopically to be monoclinic or orthorhombic, with the molecules located at sites of trivial symmetry, C1. A molecular arrangement, derived from packing considerations, which belongs to space group P21/n(C2h5), is proposed as the crystal structure.