Methyl substitution in carbenes: Lack of steric or hyperconjugative stabilization effects on the CH₃CH singlet-triplet splitting

Ab initio GVB-CI (generalized valence bond theory with configuration interaction) and MCSCF (multiconfiguration self-consistent-field) calculations have been carried out on methylene and ethylidene (CH₃CH) to investigate the effects of methyl substitution on the singlet-triplet splitting of methylene. We predict that ethylidene has a singlet-triplet splitting of 3 ± 2 kcal/mol, about 6 kcal/mol less than the value for methylene. Furthermore, our investigations show that neither steric effects nor hyperconjugative singlet stabilization effects are large enough to account for this decrease in the singlet-triplet splitting. Rather, the dominant effects of the CH₃ group are to (i) decrease p character in the nonbonding σ orbital of the triplet state, (ii) reduce electron donation to the methylenic carbon, and (iii) induce repulsions in the triplet state between the CH₃ and the singly occupied methylenic C pπ orbital. All of these effects destabilize the triplet ground state and thus decrease the singlet-triplet splitting.