Singlet-triplet gaps in substituted carbenes CXY (X, Y = H, fluoro, chloro, bromo, iodo, silyl)

Trends in the singlet-triplet state-splittings of substituted carbenes are found to be reproduced accurately with a very simple level of ab initio theory. The minimum balanced description, GVB(1/2), with a simple basis set yields singlet-triplet gaps that correlate linearly with available accurate values. This linear relationship is exploited to predict the state-splitting for the remaining members of the title series of carbenes. The magnitudes of the singlet-triplet splittings can be rationalized in terms of the charge on the carbenic carbon atom as well as π-donation from the substituents. The correlation with charge, in conjunction with electronegativity equalization, permits singlet-triplet gaps to be predicted for arbitrary simple carbenes using only a hand calculator. Since both charge and π-donation are important in determining the energy gaps, we conclude that σ-donation and π-backbonding act synergistically.