Web Release Date: March 26,
Infrared, Raman, and Inelastic Neutron Scattering Spectra of Dodecahedrane: an Ih Molecule in Th Site Symmetry
and
Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K., Department of Chemistry, University of Surrey, Guildford, Surrey, GU2 5XH, U.K., and Chemisches Laboratorium der Universität, Institute für Organische Chemie und Biochemie, 79104 Freiburg, Germany
Received: January 18, 2005
In Final Form: February 14, 2005
Abstract:
The Raman spectrum of crystalline dodecahedrane, C20H20, a species of nominal Ih symmetry, exhibits splitting of the Hg Raman active modes. The Raman inactive gerade vibrations of Gg, T1g, and T2g symmetry are found to have weak Raman activity. The IR forbidden vibrations of T2u, Gu, and Hu type have moderate IR activity. All of this is consistent with the Th site symmetry. A treatment of the structure and vibrations of dodecahedrane using a periodic lattice DFT method results in a slightly distorted Th 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 Hg 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 Å is shortened by disorder and should be revised upward to agree with the computed value of 1.558 Å.
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