Stabilized carbenes do not dimerize

Dimerization Gibbs free energies were computed for several carbenes. At the B3LYP/6-311+G(2D) level the values are smaller than at MP2/6-311+G(2D) by about 10 kcal mol(-1), the DFT results being in accord with experimental findings. The smallest dimerization energy was obtained for those compounds, which have already been synthesized (with proper substituent groups). The dimerization energy shows an excellent linear correlation with the stabilization obtained in an isodesmic reaction, giving the possibility to estimate the stability of any new nucleophilic carbene by a simple computational procedure. In this estimate, however, only electronic factors are considered, steric effects (the use of bulky protecting groups) can give additional stabilization.