Infrared,Raman,and Inelastic Neutron Scattering Spectra of Dodecahedrane:an I_h,Molecule in T_h Site Symmetry

摘要

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 A 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 Th 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 A is shortened by disorder and should be revised upward to agree with the computed value of 1.558 A.