Aromaticity Changes along the Lowest-Triplet-State Path for C=C Bond Rotation ofAnnulenyl-Substituted Olefins Probed by the Electron Localization Function

摘要

The π-contribution to the electron localization function (ELF_π) was used to analyze changes in the aromaticityof annulenyl-substituted olefins in their lowest triplet state (T_1) when the structure around the olefinbond is twisted from planar to a structure (~3p~*) at which the planes of the two RR'C units are perpendicular.The ring closure bifurcation value and the range in the bifurcation values of the ELF_π basins serveas(anti)aromaticity indicators directly linked to the electronic structure. Both Hiickel's 4n + 2 π-electron rulefor aromaticity in the singlet ground state (S_0) and Baird's 4n π-electron rule for aromaticity in the lowestππ~*tripletstate are applied. Three olefins with S_0 aromatic (T_1 antiaromatic) substituents and four olefinswith T, aromatic (S_0 antiaromatic) substituents were studied using the ELF_π topology at the OLYP/6-311G(d,p)density functional theory level. The changes in the substituent ELF_π bifurcation values upon rotation aboutthe olefin bond in the T_1 state reveal that aromatic character is recovered for the first three olefins and thatit is reduced for the latter ones. These changes in aromatic character are reflected in the shapes of the T_1potential energy surfaces as a twist away from planar structures in olefins with T_1 antiaromatic substituentsis energetically favorable, but that in olefins with T_1 aromatic substituents is unfavorable. Hence, aromaticitychange is a driver for a photochemical reaction as for many ground-state reactions.