Tetraphenyl-group IV B compounds: Flexible molecules with high symmetry crystals. I. Assignment of low frequency infrared and Raman bands

The vibrational spectroscopy of tetrephenylmethane and tetraphenyltin have been studied by Raman and far IR spectroscopy. The polarized spectra were recorded at both room temperature and low temperature (below 50 K) for oriented single crystals grown from solution. The optical quality and symmetry of these crystals, especially the unusually high (S₄) site symmetry, provides a unique oppurtunity for the study of a large, flexible molecule in a well-defined geometry and environment. Very large extinction and polarization ratios are observed for these crystals. Using the six unique Raman and two unique IR spectra, the published x-ray data, and group theory, the observed vibrational bands can be assigned symmetry types. The eight spectral experiments highly overdetermine the assignment of the vibrations to the three (A, B, and E) symmetry classes. The results show many previously unreported low frequency bands. The polarization studies show a number of near degeneracies that cannot be resolved in unpolarized experiments. There are more bands below 300 wave numbers than can be attributed to optical phonons and combination bands and there is therefore reason to believe that there is strong mixing between the molecular and lattice degrees-of-freedom.