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 (S0) or the lowest triplet state (T1) only. Thus, species with adaptive aromaticity (with aromaticity in both the S0 and T1 states) is particularly rare. On the other hand, σ-aromaticity in the T1 state has been underdeveloped, let alone adaptive σ-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive s-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 T1 state formed from out-of-plane π molecular orbital to the π* orbitals. Thus the σ-aromaticity of the three-membered ring in the S0 state could hold in the T1 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.