Anharmonicity aromaticity

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

Experimentally, there is little to suggest that large anharmonicities may be expected for the low-lying electronic states of the aromatic and aza-aromatic molecules, at least on the evidence of the extensive sequence structure and the well-developed progressions observed for the lowest excited singlet and triplet states of many molecules. 

As in the case of the aliphatic amines, secondary aromatic amines show only one NH-stretching overtone. For A-butyl aniline in CCI4, for example, the overtone is at 6675 cm" (1498 nm). The effects of hydrogen bonding on the mechanical anharmonicity of the NH-stretching vibration of N-methylaniline have been reported."... 

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