Singlet-triplet splittings and barriers to Wolff rearrangement for carbonyl carbenes.

High-level ab initio calculations at the G3(MP2)//B3-LYP level have been used to study carbomethoxychlorocarbene and related halogenocarbenes and carbonyl carbenes. Initial calculations at the more accurate W1‘ level on the subset CH2, HCCl, HCF, CCl2, and CF2 provide support for the reliability of G3(MP2)//B3-LYP for this type of problem. The W1‘ calculations also suggest that the experimental S−T splitting is slightly underestimated for HCCl and CF2 and substantially underestimated for CCl2, in keeping with other recent high-level studies. Whereas the parent carbonyl carbenes, namely formylcarbene, carbohydroxycarbene, and carbomethoxycarbene, are all predicted to have triplet ground states, their chloro and fluoro derivatives are predicted to have singlet ground states. In particular, carbomethoxychlorocarbene is predicted to have a singlet ground state, with the singlet−triplet splitting estimated as −16.0 kJ mol-1. The barriers to Wolff rearrangement of the singlet carbonyl carbenes generally (but no...