Infrared, Raman, and inelastic neutron scattering spectra of dodecahedrane: An I _h molecule in T _h site symmetry

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.