Resonance structures of N-heterocyclic carbenes

Electronic structure of N-heterocyclic carbenes with five-member rings and two nitrogen atoms was studied using the natural resonance theory (NRT) in the framework of density functional theory. Also the role of the hydroxyl substituents on the carbon atoms was investigated. The analysis provides a consistent picture of chemical bonding, even in cases where no single Lewis structure can be drawn. It interprets the observed stabilities and aromaticities of the different isomers. A carbene lone pair on C1 atom was identified in all cases, while the electron donation to the formally empty pz orbital of this atom was found to be a crucial effect in all cases.