Far-infrared spectra of crystalline acetylenes C₂H₂ and C₂D₂

The far-infrared spectra of polycrystalline films of acetylene and fully deuterated acetylene have been recorded at 77°K, using a commercial Michelson interferometer. Two sharp bands are observed for each crystal, and the frequency change on deuteration leads to their assignment as translational lattice modes. From these and other spectroscopic results, deductions concerning the crystal structure, which has not been directly measured at this temperature, are made, and it is concluded that the most likely factor group is of DM symmetry with two molecules per unit cell on CM sites, a structure similar to those of crystalline carbon disulfide and the halogens.