BCSJ : Vol. 78 (2005) , No. 7 pp.1230-1240

Molecular Symmetry Group as Applied to the Spectroscopy of Molecules with Methyl Groups: Toluene, Nitromethane, and 5-Methyltropolone’s

Nobuaki Kanamaru¹⁾

1) Department of Chemistry, Faculty of Science, Nagoya University

(Received January 5, 2005)

A theory of the molecular symmetry group (MSG) is presented to describe the spectroscopy of molecules that consist of methyl (top) and planar (frame) groups and belong to the G₁₂ MSG. This was done in a form of Fourier-series expansion on the torsional angle of the methyl group concerning its two kinds of coupling terms, by choosing three kinds of systems in the title: (1) A morphing technique to describe the vibronic coupling is applied to interpret the torsionally induced S₁ ← S₀ absorption spectra of toluene (as partially described in a recent Letter). (2) The torsion–vibration coupling term in the top (through its interaction with the frame) was confirmed to split the degenerate e′ levels of either vibrational or torsional modes into either a₁″ + a₂″ levels/modes or a₁′ + a₂′ levels, respectively. The latter was made with the interpretation of either broad IR absorption spectra of nitromethane or peculiar low-frequency bands in the electronic spectra of 5-methyltropolone, respectively.