RIGOROUS INTERPRETATION OF ELECTRONIC WAVE-FUNCTIONS .4. ORIGINS OF THE UNUSUAL STABILITY OF THE 1,3-DIMETHYLIMIDAZOL-2-YLIDENE CARBENE

Ab initio electronic structure calculations, carried out in conjunctio n with rigorous analysis of the resulting wavefunctions, demonstrate t hat the electronic factors responsible for the unusual stability of th e 1,3-dimethylimidazol-2-ylidene carbene are directly related to the s ubstantial sigma-backdonation from the carbenic carbon to the adjacent nitrogen atoms. The pi-donation is found to play only a minor role, r uling out the presence of significant stabilization due to ylidic reso nance structures. The unusually large proton affinity of the carbene i s explained by the extra pi-electron stabilization of the correspondin g 1,3-dimethylimidazolium cation. The MP2/6-311++G* level of theory i s found to be sufficient for accurate predictions of the geometries of the imidazol-2-ylidene carbenes. (C) 1993 John Wiley & Sons, Inc.