Barrier to methyl internal rotation of 1-methylvinoxy radical in the (X)over-tilde((2)A '') and (B)over-tilde((2)A '') states: Experiment and theory

The jet-cooled laser induced fluorescence spectrum of the (B) over tilde <- - <(X)over tilde> electronic transition of the 1-methylvinoxy radical is as signed, including both hot and cold bands. The barrier to methyl internal r otation in both (X) over tilde and (B) over tilde states is determined by f itting pure torsional transitions to a one-dimensional hindered-rotor model . The resulting 3-fold torsional barrier parameters are V-3' = -740 +/- 30 cm(-1) for the (B) over tilde state (minimum-energy conformation with one m ethyl CH bond cis to the frame CO bond) and V-3" = +130 +/- 30 cm(-1) for t he (X) over tilde state (methyl CH bond trans to CO). The intensity pattern clearly indicates a change in the preferred methyl conformation upon excit ation, while ab initio calculations provide the absolute conformations in e ach state, A variety of ab initio methods including CASSCF, multireference CI, and coupled-cluster techniques were applied to both the (X) over tilde and the (B) over tilde states of 1-methylvinoxy. Only the largest coupled-c luster calculations yield a (B) over tilde -state barrier in good quantitat ive agreement with experiment. In unsubstituted vinery, a (B) over tilde -s tate geometry adjusted earlier to fit experimental rotational constants (re f 10) is evidently in error.