Aromaticity changes along the lowest-triplet-state path for C=C bond rotation of annulenyl-substituted olefins probed by the electron localization function.
The pi-contribution to the electron localization function (ELF(pi)) was used to analyze changes in the aromaticity of annulenyl-substituted olefins in their lowest triplet state (T1) when the structure around the olefin C=C bond 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(pi) basins serve as (anti)aromaticity indicators directly linked to the electronic structure. Both Hückel's 4n + 2 pi-electron rule for aromaticity in the singlet ground state (S0) and Baird's 4n pi-electron rule for aromaticity in the lowest pipi* triplet state are applied. Three olefins with S0 aromatic (T1 antiaromatic) substituents and four olefins with T1 aromatic (S0 antiaromatic) substituents were studied using the ELF(pi) topology at the OLYP/6-311G(d,p) density functional theory level. The changes in the substituent ELF(pi) bifurcation values upon rotation about the olefin bond in the T1 state reveal that aromatic character is recovered for the first three olefins and that it is reduced for the latter ones. These changes in aromatic character are reflected in the shapes of the T1 potential energy surfaces as a twist away from planar structures in olefins with T1 antiaromatic substituents is energetically favorable, but that in olefins with T1 aromatic substituents is unfavorable. Hence, aromaticity change is a driver for a photochemical reaction as for many ground-state reactions.
Department of Biochemistry and Organic Chemistry, Box 576, Uppsala University, 751 23 Uppsala, Sweden.
Sébastien
S
Villaume
Henrik
H
Ottosson
D000475:Alkenes / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry*; D004583:Electrons; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D010777:Photochemistry; D011789:Quantum Theory; D012399:Rotation
D000475:Alkenes; D006841:Hydrocarbons, Aromatic; D002244:Carbon