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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry