Singlet carbene stability: linear free energy analysis of substituent effects

The energies of a series of substituted methylene and methylcarbene singlet states were computed by quantum mechanical calculations at the CCSD(T)/cc-pVDZ//M06-2X/cc-pVDZ level. Stabilization energies relative to singlet methylene were correlated with polarizability, polar and resonance Hammett sigma constants using multiple regression analysis. The results revealed that most of the carbenes gave rise to inverse polar effects. It is concluded that separation of polar and resonance effects via linear free energy methods (LFE) fails for singlet carbenes with substituents directly attached to carbene carbons.