The Raman spectrum of crystalline dodecahedrane, C 20H 20, a species of nominal I h symmetry, exhibits splitting of the H g Raman active modes. The Raman inactive gerade vibrations of G g, T 1g, and T 2g symmetry are found to have weak Raman activity. The IR forbidden vibrations of T 2u, G u, and H u type have moderate IR activity. All of this is consistent with the T h site symmetry. A treatment of the structure and vibrations of dodecahedrane using a periodic lattice DFT method results in a slightly distorted T h structure with six C-C bonds that are 0.001 Å longer than the other 24. The vibrational spectrum computed for this structure exhibits splittings of the H g modes that are consistent with the observed spectra, but the computed splittings are larger than observed in room-temperature data. A complex pattern observed in the C-H stretching region is assigned. The inelastic neutron scattering spectrum calculated from the computed normal modes for the T h molecule in the lattice agrees quantitatively with experiment when overtone and combination transitions are included and allowance is made for anharmonicity of the C-H stretch motion. Finally, it is argued that the existing crystallographic determination of the average C-C bond length of 1.544 Å is shortened by disorder and should be revised upward to agree with the computed value of 1.558 Å.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry