Barrier to Methyl Internal Rotation of Cis- and <i>Trans</i>-2-Methylvinoxy Radicals in the X(²A‘ ‘) and B(²A‘ ‘) States:  Experiment and Theory

Abstract

The jet-cooled laser induced fluorescence spectrum of the B̃ ← X̃ electronic transition of 2-methylvinoxy radical is assigned as a superposition of contributions from noninteracting cis and trans isomers. The spectrum of the cis isomer is identified by comparison with ab initio electronic structure calculations; both theory and experiment clearly indicate that the methyl conformation changes from the X̃ state to the B̃ state. Fits of both hot and cold bands to a one-dimensional torsional model yield methyl rotor barrier magnitudes of 270 ± 20 cm -1 in the X̃ state and 200 ± 20 cm -1 in the B̃ state. The ab initio calculations show that in the ground state the preferred conformation places one methyl CH bond in the plane of the molecular frame cis to the vicinal CC bond. Assignment of the spectrum of trans -2-methylvinoxy is more tentative because no resolved hot bands are available to corroborate the model. Our best estimate for the B̃ state barrier magnitude is 60 ± 15 cm -1 . Multireference configuration interaction calculations and coupled cluster calculations are reasonably successful in obtaining methyl torsional barriers in agreement with experiment, although high accuracy is elusive for the B̃ state of both cis and trans isomers. By comparison with simpler cases, we infer that the π radical character of the B̃ state strongly influences the methyl torsional barrier.