Characteristics of the electronic structures of diabatically and adiabatically Z/E-isomerizing olefins in the T-1 state

Nonlocal gradient-corrected and hybrid density functional theory (DFT) have been used to calculate T1 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-prop enyl)benzene (5), 1,3-divinylbenzene (6), and 2-(1-propenyl)anthracene (7). Calculated properties were used to determine differences in electronic str ucture of olefins that follow adiabatic vs diabatic Z/E-isomerization mecha nisms. In the planar TI structure. the C=C bond in 1 is elongated to a sing le bond, but in 7 it remains a double bond, archetypal of excitations in th e olefinic bond and in the substituent, respectively. Changes in geometries and spin-density distributions of 2-7 reveal that substituent aromaticitie s vary along the Tl PES. For systems that isomerize diabatically (e.g., 2), substituent aromaticity is regained in the 90 degrees twisted structure of the C=C bond (3p*). This leads to stabilization and a minimum on the PES a t 3p*. If the substituent of the planar T1 olefin fully can accommodate the triplet biradical and still remain aromatic as in 7, aromaticity is instea d reduced upon twist to 3p*, SO that the T1 PES has a barrier that is suita ble for adiabatic isomerizations. The planar structures of olefins with sub stituents that are partially antiaromatic in Ti (e.g., phenyl) can be stabi lized by radical accepting groups in the proper positions (e.g., 5). In sum mary, our calculations indicate that for an aryl-substituted olefin the str ucture with the highest substituent aromaticity in TI corresponds to the mi nimum on the TI PES of Z/E-isomerizations.