Lattice Vibrations of Solid Cyanogen

Abstract

The infrared and Raman spectra of the lattice vibration region of solid cyanogen are reported. All six of the pseudotranslational modes permitted by symmetry are observed in the infrared. In the Raman, seven distinct bands are observed, compared with the eight symmetry‐allowed pseudorotational modes. A Lennard‐Jones, quadrupole–quadrupole intermolecular potential of the type suggested by Walmsley and Pople leads to calculated lattice‐mode frequencies which are in only moderately good agreement with the observed. On the other hand, a quite satisfactory fit of all of the observed frequencies is obtained by arbitrary adjustment of a single parameter $\epsilon$ in a crystal potential consisting of 6–12 interactions between pairs of nitrogen atoms. Infrared intensities calculated from a quadrupole‐induced intensity model and Raman intensities derived from radiation field induced moments agree reasonably well with observed intensities. The atom–atom potential is also applied to solid CO₂ and satisfactorily predicts the lattice mode frequencies.