Jet-cooled laser-induced fluorescence spectra of the B̃ ← X̃ electronic transition of 1-methylvinoxy and of a cis/trans mixture of 2-methylvinoxy are presented. We observe some 50 vibronic bands in the spectrum of 1-methylvinoxy. The complexity is due to a change in the preferred methyl rotor orientation on electronic excitation. The B̃-state barrier to internal rotation is approximately 750 cm-1. The B̃-state fluorescence lifetimes decrease from about 130 ns near the origin to 26 ns for internal energy of 2700 cm-1. We observe some 22 vibronic bands in the spectrum of the cis/trans mixture of 2-methylvinoxy. The B̃-state fluorescence lifetimes decrease gradually from about 190 ns near the origin to 140 ns for internal energy of 1170 cm-1 and then very sharply at higher energy. The new spectra serve as fingerprints for identification of specific methylvinoxy isomers as products of chemical reactions of O(3P) with alkenes, as recently observed by Bersohn and co-workers.

The jet-cooled laser induced fluorescence spectrum of the B̃ ← X̃ electronic transition of the 1-methylvinoxy radical is assigned, including both hot and cold bands. The barrier to methyl internal rotation in both X̃ and B̃ states is determined by fitting 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 B̃ state (minimum-energy conformation with one methyl CH bond cis to the frame CO bond) and V 3‘ ‘ = +130 ± 30 cm-1 for the X̃ state (methyl CH bond trans to CO). The intensity pattern clearly indicates a change in the preferred methyl conformation upon excitation, while ab initio calculations provide the absolute conformations in each state. A variety of ab initio methods including CASSCF, multireference CI, and coupled-cluster techniques were applied to both the X̃ and the B̃ states of 1-methylvinoxy. Only the largest coupled-cluster calculations yield a B̃-state barrier in good quantitative agreement with experiment. In unsubstituted vinoxy, a B̃-state geometry adjusted earlier to fit experimental rotational constants (ref ) is evidently in error.

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.