A Theoretical Spectroscopic Study of the Ã1Au(S1) ← X̃1Ag(S0), n → π* Transition in Biacetyl, (CH3CO)2

Abstract The S 0 - S 1 electronic band spectrum in biacetyl arising from the activity of the methyl torsion modes was simulated from RHF/UHF ab initio calculations. A fit of the calculated band spectrum to the cold-jet fluorescence excitation spectrum provided an assignment of the major bands and the location of the system origin to the weak band at 22 182 cm −1 . The analysis of the warm-jet excitation spectrum revealed, that for the S 0 state, the interaction between ν 15 ( a u ) gearing and ν 21 ( b g ) antigearing modes is very small and depends mainly on the kinetic energy coupling. For the S 1 excited state, the gearing and antigearing modes are widely separated as a result of a significant sine-sine potential energy coupling term. The differences in the gearing-antigearing coupling was attributed to the increased rigidity of the (CO) 2 heavy atom frame that may transmit more easily the coupling, as well as the increased flexibility in the methyl wagging coordinate in the S 1 upper state due to the n → π* electron excitation.