Characteristics of the Electronic Structures of Diabatically and Adiabatically Z/E-Isomerizing Olefins in the T1 State

Nonlocal gradient-corrected and hybrid density functional theory (DFT) have been used to calculate T 1 potential energy surfaces (PES), spin densities, and geometries of ethylene and aromatic olefins of various sizes: ethylene ( 1 ), styrene ( 2 ), stilbene ( 3 ), 1,1-diphenylethylene ( 4 ), 1,4-bis-(1-propenyl)benzene ( 5 ), 1,3-divinylbenzene ( 6 ), and 2-(1-propenyl)anthracene ( 7 ). Calculated properties were used to determine differences in electronic structure of olefins that follow adiabatic vs diabatic Z/E -isomerization mechanisms. In the planar T 1 structure, the C C bond in 1 is elongated to a single bond, but in 7 it remains a double bond, archetypal of excitations in the olefinic bond and in the substituent, respectively. Changes in geometries and spin-density distributions of 2 − 7 reveal that substituent aromaticities vary along the T 1 PES. For systems that isomerize diabatically (e.g., 2 ), substituent aromaticity is regained in the 90° twisted structure of the C C bond ( 3 p*). This leads to stabilization and a minimum on the PES at 3 p*. If the substituent of the planar T 1 olefin fully can accommodate the triplet biradical and still remain aromatic as in 7, aromaticity is instead reduced upon twist to 3 p*, so that the T 1 PES has a barrier that is suitable for adiabatic isomerizations. The planar structures of olefins with substituents that are partially antiaromatic in T 1 (e.g., phenyl) can be stabilized by radical accepting groups in the proper positions (e.g., 5 ). In summary, our calculations indicate that for an aryl-substituted olefin the structure with the highest substituent aromaticity in T 1 corresponds to the minimum on the T 1 PES of Z/E -isomerizations.