Theoretical designs for triplet ground-state carbenes: A target for experimental studies

A density functional study of triplet ground-state carbenes was carried out using the B3LYP/6-311G(d) and B3LYP/cc-PVTZ_DK methods. The structures of 10 model carbenes in their closed-shell singlet and open-shell triplet states were obtained. Both a valence electron model and our quantum calculations strongly suggest that a persistent triplet ground-state carbene occurs when the carbenic center is attached to two bulky substituents arranged in a perpendicular conformation. If the attached substituents bear aromatic rings, then their ortho positions must be occupied by other protecting groups. Moreover, if the attached atoms are less electronegative than the carbon atom, this can also lead to an increase in the singlet-triplet energy gap, and thus to further stabilization of the triplet state. The results obtained allow a number of predictions to be made.