OVERTONE LINE NARROWING AND INTRAMOLECULAR VIBRATIONAL-ENERGY REDISTRIBUTION IN SUBSTITUTED TOLUENES

The vibrational overtone spectra of some substituted toluenes are presented. The aryl CH stretching overtone regions have a simple local mode interpretation. Distinct aryl CH stretching overtone progressions can be identified for each structurally nonequivalent type of aryl oscillator. The methyl CH stretching overtone regions are more complex. Both conformationally nonequivalent methyl CH bonds and unresolved methyl rotational structure may contribute to the methyl spectral profiles. An additional factor involves a strong alkyl CH stretch-bend Fermi resonance. Two of the molecules studied, 2,3,5,6-tetrafluorotoluene and 2,3,4,5,6-pentafluorotoluene, stand out. The DELTA-upsilon-CH = 3 methyl overtone linewidths of these molecules are markedly reduced compared to the other toluenes studied and reveal underlying structure. The narrowing is interpreted as arising from a detuning of the alkyl CH stretch-bend Fermi resonance due to the presence of multiple fluorine substituents.