A quest for stable bicyclic carbenes with one, two, and three carbenic centers at theoretical level.

We have scrutinized eighteen new derivatives of carbenes with one, two, and three carbenic centers and compared their structural and thermodynamic parameters, at B3LYP/6-311++G** level of theory. All novel carbene scrutinized show singlet ground stat, and the highest stability belongs to 1,2,3,4-tetraazabicyclo[1.1.1]-5-pentanylene (9) (ΔEs-t = 47.95 kcal/mol). This stability can be related to their σ−bond and intermolecular interactions. This bond arose from a tendency of nonbonding electrons of nitrogen (N(E or G)) to empty p orbital of the carbenic center. In addition, carbene 16s and 17s because of three coordinate covalent bonds with their carbenic centers have low nucleophilicity (N). The increase in band gaps (ΔΕHOMO–LUMO) is correlated with an increase in ΔΕs–t values. The purpose of the present work was to assess the influence of 1 to 4 nitrogen substituents on the stability (ΔEs-t), ΔΕHOMO–LUMO, N, electrophilicity (ω), and isodesmic reactions. Finally, our investigation introduces novel carbenes that can be applied as cumulated multi-dentate ligands. [ABSTRACT FROM AUTHOR]