Raman phonon spectra of partially-deuterated crystalline ethylenes

• Published in: The Journal of chemical physics 1978-01, Vol.68 (5), p.2151-2158
• Main Authors: Whitfield, R. G., Leroi, G. E.
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.436038

The Raman phonon spectra of C2H3D, cis-C2H2D2, trans-C2H2D2, 1,1-C2H2D2, and C2HD3 have been obtained, and various sets of atom–atom interaction potentials were used to calculate the lattice frequencies of the C2H4 crystal using recent single crystal x-ray data. The ’’best’’ interaction potential was used to calculate the librational frequencies of the partially-deuterated ethylenes, assuming that the crystal structure and symmetry is unchanged by deuteration. From a comparison of the observed and calculated frequencies, it was determined that the lattice modes of these crystals are amalgamated, and can be described in the virtual crystal limit. Consistent with the virtual crystal theory, mutual exclusion was retained for the lattice modes of these disordered solids, illustrating the ’’effective’’ Ci site symmetry. Slight changes in the relative intensities of the observed librations were noted, and the experimentally determined perturbation strengths were found to be in agreement with those calculated using the virtual crystal approximation. An attempt to observe the ’’missing’’ optically active translation of C2H4 in the far infrared is described; it is suggested to be at ∼35 cm−1.