Ab initio molecular orbital study of the weak C̃2A′←X̃2A′ transition of the vinyl radical

The oscillator strength was calculated for the vinyl radical at MRCISD and CASSCF levels between the ground state and several doublet excited states. Results indicated that the C̃2A′←X̃2A′ transition, of π*←π character that was usually thought to be strong, was very weak. The C̃2A′ state is the result of coupling between the π*←π triplet excitation and the nonbonding electron, and the transition is essentially ‘spin-forbidden’. Another high-energy doublet state was found with strong transition from the ground state arising from coupling between the π*←π singlet excitation and the nonbonding electron. This appears to be a common feature for π*←π excitation in σ radicals.