Interpretation of the vibrational spectra of methylglyoxal and biacetyl in their first singlet excited electronic states

Laser-induced fluorescence spectra for the first allowed electronic transition (22125 cm^?1) of methylglyoxal (CH₃COCHO) and its perdeutero analog (CD₃COCDO) in a supersonic nozzle beam are quantitatively represented assuming that the potential function governing the CH₃(CD₃) rotation is changed during the transition. In the excited state the potential function is ternary (V₁ = 95 (1 + cos 3θ)cm^?1) as in the fundamental state (V₀ = 134.5 (1 - cos 3θ)cm^?1), but the minima are shifted by an angle of π/3. The spectrum of biacetyl (CH₃COCH₃CO) can be reproduced assuming two uncoupled methyl groups undergoing similar conformational changes during the electronic transition (the estimated potential function is V₁ = 117.5 (1 + cos 3θ) cm^?1 for each methyl group), in perfect agreement with the most recent assignment of the 0-0 transition. These results are consistent with ab initio calculations for the fundamental and first excited singlet states.