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 (E-u) and reaction energies (DeltaE) for the dimerizations of imidazol-2-ylidenes are larger than those of imidazolin-2-ylidenes. It was observed that E-a of dimerization is approximately proportional to the singlet-triplet energy separation (DeltaE(ST)), aromatic stabilization energy (ASE), and DeltaE of the carbene. Excellent linear correlation is seen between E-a and ASE. Contrary to previous suggestions, we found that 4,5-dichloro substitutions decrease the stability of imidazol(in)-2-ylidenes. Steric effects on E-a occur noticeably as isopropyl (i-Pr) substitutions are introduced to the carbenes. (C) 2001 Elsevier Science B.V. All rights reserved.