A THEORETICAL SPECTROSCOPIC STUDY OF THE A)OVER-TILDE1AU(S1)[-XOVER-TILDE-ASTERISK-1AG(S0), N-]PI-ASTERISK TRANSITION IN BIACETYL, (CH3CO)2

The S0-S1 electronic band spectrum in biacetyl arising from the activi ty of the methyl torsion modes was simulated from RHF/UHF ab initio ca lculations. A fit of the calculated band spectrum to the cold-jet fluo rescence 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 S0 state, the interaction between nu15(a(u)) gearing and nu21 (b(g )) antigearing modes is very small and depends mainly on the kinetic e nergy coupling. For the S1 excited state, the gearing and antigearing modes are widely separated as a result of a significant sine-sine pote ntial 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 S1 upper state due to the n --> pi electron excitation. (C) 1994 Academic Pre ss, Inc.