Motions of Molecules in Condensed Systems. XI. Infrared Spectrum and Structure of a Single Crystal of Ethylene

Abstract

The infrared absorption spectra of thin sections of single crystals of ethylene have been recorded in the frequency range between 650 and 3600 cm−1, using polarized infrared radiation. Comparisons are made with similarly recorded spectra of the liquid and vapor phases. A complete assignment of the absorption bands is given. The frequencies of some primary lattice modes of the molecule in the crystal, and of some inactive molecular modes, are inferred from combinations. The ethylene crystal is found spectroscopically to be monoclinic (C2h) or orthorhombic (D2h), with the molecules located at sites of Ci symmetry. Using the known lattice of the carbon atoms, a molecular arrangement, described by space group P 21/n (C2h5), is derived as the true crystal structure.