Characterization of Carbenes via Hydrogenation Energies, Stability, and Reactivity: What's in a Name?

The hydrogenation energies and singlet-triplet (S-T) splittings at the G3MP2 level of theory have been calculated for a wide range of carbenes. The carbene, :CXY with different substituents (X, Y=H, CN, NC, F, Cl, OH, OCH3 , CH3 , CF3 , SiH3 , SiMe3 , phenyl, CH=O, PH2 , and NH2 ) at the carbenic carbon center, immidazole-based carbenes, Bertrand's carbenes, and Seppelt's CF3 CSF3 were studied. The stable carbenes are singlets with large S-T splittings and with the least exothermic hydrogenation energies. The singlet ground state immidazole-based carbenes are calculated to have the least exothermic hydrogenation energies (-15 to -30 kcal mol-1 ) and the largest S-T gaps. The singlet ground states of the Bertrand carbenes have more exothermic hydrogenation (ca. -67 kcal mol-1 ) energies and much smaller S-T gaps. The more exothermic reaction energies arise due to the need to make the phosphorus planar in the carbene so that it can donate into the empty p-orbital at the carbene carbon center. The bending potential at the carbene carbon center in the Bertrand compounds is very flat with a large XC:Y angle. Seppelt's CF3 CSF3 appears to be energetically similar to the Bertrand system, probably due to the required adjustments for geometric distortions at the sulfur center.