Abstract
Two quanta of the aldehyde H-wag vibrational mode are observed with relatively constant activity in the vibrational structure of 3A″ (nπ*) → 1A′ phosphorescence spectra of benzaldehyde, deuterated benzaldehydes, pyridine aldehydes, p-methylbenzaldehyde, p-fluorobenzaldehyde, and p-chlorobenzaldehyde at 4.2°K in a methylcyclohexane polycrystalline environment. The fundamental is either missing or very weak. Contrasted to the aldehyde H-wag mode, higher quanta of the CHO torsional mode are observed in the phosphorescence of the same molecules with variable intensity and progression length. These results are interpreted in terms of a static (i.e., valence) distortion of the 3nπ potential surface along the aldehyde H-wagging coordinate, and pseudo-Jahn-Teller distortion along the CHO torsional coordinate. The valence distortion is analogous to a distortion found for the 3nπ* state in propynal and its origin probably lies in excess electron density at the carbonyl carbon in the excited state. The dynamical Jahn-Teller distortion is ascribed to vibronic interaction between 3A″ (nπ) and the nearest 3A′(nπ*) states.
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