Infrared, Raman, and inelastic neutron scattering spectra of dodecahedrane:: an Ih molecule in Th site symmetry

The Raman spectrum of crystalline dodecahedrane, C20H20, 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 angstrom 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 angstrom is shortened by disorder and should be revised upward to agree with the computed value of 1.558 angstrom.