Substituted Hammick carbenes: The effects of fused rings and hetero atoms through DFT calculations

Abstract

AbstractEffects of fused aromatic rings are probed on the stability of 42 novel singlet (s) and triplet (t) Hammick carbenes at density functional theory (DFT). Results show that (a) singlet‐triplet energy gaps (ΔΕs‐t) emerge with positive values indicating that every singlet carbene as ground state is more stable than its corresponding triplet; (b) all species turn out as minima for exhibiting no negative force constant; (c) from both thermodynamic and kinetic viewpoints, the highest stability is demonstrated by carbenes situated between two heteroatoms of fused five‐membered rings, in a “W” arrangement with showing the highest ΔΕs‐t, and the widest band gaps (ΔΕHOMO‐LUMO); thus, there is a consistency between the ΔΕs‐t and ΔΕHOMO‐LUMO values; (d) regardless of how orchestrated, the order of stabilization for fused rings is pyrrole > furan > thiophene; (e) fusion of two 5‐membered rings, in a given arrangement, have more stabilizing effect than one; (f) while five‐membered heterocyclic rings such as pyrrole stabilizes the carbenes, pyridine and benzene rings destabilize them; (g) Pyridines destabilize carbenes more than benzene rings; (h) size, type, orientation, and the number of fused rings have pronounced effect on the stability of Hammick carbenes; (i) major criteria is used to compare and contrast relative stability of the studied species include: resistance to dimerization, ΔΕs‐t, and ΔΕHOMO‐LUMO; (j) computational surveys are waiting for experimental testing and verifications that place the nitrogen lone pair in the plane of the ring in a favorable spatial position to act not only as donors, but also acceptors of π‐electron density.