Theory predicts triplet ground-state carbene containing the N-heterocyclic carbenic unit

Abstract

A theoretical study of singlet- and triplet-state cyclic vinylidenes was performed using the B3LYP, B3PW91, CCSD, CCSD(T), QCISD, and QCISD(T) methods at the 6-311G(d), 6-311++G(d,p), 6-311++G(df,pd), 6-311++G(2df,2pd), and 6-311++G(3df,3pd) basis sets. Fifty-eight vinylidenes, featuring the N-heterocyclic carbenic (NHC) unit, were chosen as the model molecules for this study. The computations demonstrate that fifty-one cyclic vinylidenes are predicted to have a triplet ground state with a singlet and triplet splitting energy of approximately −0.11 to −20 and −1.8 to −21 kcal/mol, for the B3LYP/6-311++G(d,p) and B3PW91/6-311++G(df,pd) levels of theory. Our theoretical findings suggest that it is the π-electron-rich ability of the NHC ring that makes cyclic vinylidene preferably adopt the triplet ground state. A valence electron model was used to explain the computational results. The theoretical observations strongly suggest that besides the traditional fact that the carbenic center is attached by two sterically bulky substituents, the use of an NHC skeleton can significantly increase the probability of producing a vinylidene that has a triplet ground state.