Computational study on the stability of imidazol-2-ylidenes and imidazolin-2-ylidenes

The dimerization reactions of 12 cyclic diaminocarbenes have been studied using density functional theory (DFT). The activation energies (Ea) and reaction energies (ΔE) for the dimerizations of imidazol-2-ylidenes are larger than those of imidazolin-2-ylidenes. It was observed that Ea of dimerization is approximately proportional to the singlet–triplet energy separation (ΔES–T), aromatic stabilization energy (ASE), and ΔE of the carbene. Excellent linear correlation is seen between Ea and ASE. Contrary to previous suggestions, we found that 4,5-dichloro substitutions decrease the stability of imidazol(in)-2-ylidenes. Steric effects on Ea occur noticeably as isopropyl (i-Pr) substitutions are introduced to the carbenes.