Far-Infrared Spectra of Ethane, Ethylene, and Acetylene

The far-infrared spectra of crystalline C2H6, C2D6, C2H4, C2D4, C2H2, and C2D2 have been recorded at various temperatures between 25 and 140°K. Each compound showed two absorption bands which were shown to be due to translational lattice modes. Upon heating, these bands broadened and decreased in intensity; they were also displaced toward lower frequencies by about 0.1 cm−1/°K. For ethylene an analysis was carried out based on the “diatomic potential.” Here each molecule was assumed to consist of a pair of centers separated by an effective distance d, each center interacting, through a Lennard-Jones-type potential, with the centers in all neighboring molecules. Frequencies and thermal frequency shifts were calculated as a function of d, and good agreement was attained with the observed values for ethylene, with d = 1.04 Å.