Adaptive σ-Aromaticity in an Unsaturated Three-Membered Ring

Based on Huckel's and Baird's rules, species are aromatic either in the lowest singlet state (S-0) or the lowest triplet state (T-1) only. Thus, species with adaptive aromaticity (with aromaticity in both the S-0 and T-1 states) is particularly rare. On the other hand, sigma-aromaticity in the T(1)state has been underdeveloped, let alone adaptive sigma-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive sigma-aromaticity in an unsaturated three-membered ring, which is a traditional area dominated by pi-aromaticity. The origin of adaptive sigma-aromaticity could be attributed to the excitation mode of the T(1)state formed from out-of-plane pi molecular orbital to the pi* orbitals. Thus the sigma-aromaticity of the three-membered ring in the S-0 state could hold in the T-1 state. Our findings extend the concept of adaptive sigma-aromaticity into three-membered rings and could be useful to further develop the concept of both sigma-aromaticity and adaptive aromaticity.