| Abstract
| - A mechanism for the photochemical conversion of 2-vinyl-1,3-terphenyl to 8,9a-dihydrophenanthrene (Lewis, F. D.; Zuo, X.; Gevorgyan, V.; Rubin, M. J. Am. Chem. Soc.2002, 124, 13664−13665) ispresented in this study, based on ab initio restricted active space self-consistent field calculations and amolecular mechanics−valence bond dynamics simulation of a model system: the syn isomer of2-vinylbiphenyl. An extended crossing seam between the ground and first excited electronic states wasfound to be largely responsible for the efficient photocyclization of the photochemically active syn isomer.This mechanism is nonadiabatic in nature, with an excited-state reaction pathway approaching the crossingregion during the initial stage of cyclization. Dynamics simulation shows that this seam is easily accessibleby vibrational motion in the branching space, once a small barrier is passed on the S1 excited-state potentialenergy surface. Ultrafast radiationless decay to the ground state then follows, and the cyclization is completedon this surface. A second possible mechanism was identified, which involves complete adiabatic cyclizationon the S1 surface, with decay to the ground state (at a different conical intersection) only taking place oncethe product is formed. Thus, there is a competition between these two mechanismsnonadiabatic andadiabaticgoverned by the dynamics of the system. A large quantum yield is predicted for thephotocyclization of the syn isomer of 2-vinylbiphenyl and 2-vinyl-1,3-terphenyl, in agreement withexperimental observations.
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