Barrier to methyl internal rotation of cis- and trans-2-methylvinoxy radicals 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 2-methylvinoxy radical is assigne d as a superposition of contributions from noninteracting cis and trans iso mers. The spectrum of the cis isomer is identified by comparison with ab in itio electronic structure calculations; both theory and experiment clearly indicate that the methyl conformation changes from the (X) over tilde state to the (B) over tilde state. Fits of both hot and cold bands to a one-dime nsional torsional model yield methyl rotor barrier magnitudes of 270 +/- 20 cm(-1) in the (X) over tilde state and 200 +/- 20 cm(-1) in the (B) over t ilde state. The ab initio calculations show that in the ground state the pr eferred conformation places one methyl CH bond in the plane of the molecula r frame cis to the vicinal CC bond. Assignment of the spectrum of trans-2-m ethylvinoxy is more tentative because no resolved hot bands are available t o corroborate the model. Our best estimate for the (B) over tilde state bar rier magnitude is 60 +/- 15 cm(-1). Multireference configuration interactio n calculations and coupled cluster calculations are reasonably successful i n obtaining methyl torsional barriers in agreement with experiment, althoug h high accuracy is elusive for the B state of both cis and trans isomers. B y comparison with simpler cases, we infer that the Jr radical character of the B state strongly influences the methyl torsional barrier.