Infrared Spectra of Crystalline C2H2, C2HD, and C2D2

Abstract

The infrared absorption spectra of polycrystalline films of C2H2, C2HD, and C2D2 have been examined at 63°K in the frequency range between 4500 and 450 cm—1. The spectra of the solid state and the gas phase of the various acetylenes are compared and the appropriate vibrational assignments are made. Several absorptions attributable to combinations of lattice modes and molecular fundamentals were found. The observed multiplet structure of the fundamental vibrations indicates that in the low‐temperature modification of C2H2 and C2D2 the molecules are located at sites of C2h symmetry. Spectra of various solid solutions composed of mixtures of isotopic species were obtained and their significance discussed. The effect of isotopic substitution on the splittings in the ν5 region led to the conclusion that the dipole—dipole coupling model is sufficient to explain the observed spectra.