Adaptive σ-Aromaticity in an Unsaturated Three-Membered Ring.

Based on Hückel's and Baird's rules, species are aromatic either in the lowest singlet state (S ) or the lowest triplet state (T ) only. Thus, species with adaptive aromaticity (with aromaticity in both the S and T states) is particularly rare. On the other hand, σ-aromaticity in the T state has been underdeveloped, let alone adaptive σ-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive σ-aromaticity in an unsaturated three-membered ring, which is a traditional area dominated by π-aromaticity. The origin of adaptive σ-aromaticity could be attributed to the excitation mode of the T state formed from out-of-plane π molecular orbital to the π* orbitals. Thus the σ-aromaticity of the three-membered ring in the S state could hold in the T state. Our findings extend the concept of adaptive σ-aromaticity into three-membered rings and could be useful to further develop the concept of both σ-aromaticity and adaptive aromaticity.