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Cyclooctatetraene Computational Photo- and Thermal Chemistry:  A Reactivity Model for Conjugated Hydrocarbons

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journal contribution
posted on 17.10.2002 by Marco Garavelli, Fernando Bernardi, Alessandro Cembran, Obis Castaño, Luis Manuel Frutos, Manuela Merchán, Massimo Olivucci
We use ab initio CASSCF and CASPT2 computations to construct the composite multistate relaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that an efficient population of the dark excited state (S1) takes place after ultrafast decay from the spectroscopic excited state (S2). A planar D8h-symmetric minimum represents the collecting point on S1. Nonadiabatic transitions to S0 appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S1 minimum following specific excited-state reaction paths. The higher-energy conical intersection belongs to the same type of intersections previously documented in linear and cyclic conjugated hydrocarbons and features a triangular −(CH)3− kink. This point mediates both cis → trans photoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-bond shifting. The mapping of both photochemical and thermal reaction paths (including also Cope rearrangements, valence isomerizations, ring inversions, and double-bond shifting) has allowed us to draw a comprehensive reactivity scheme for cyclooctatetraene, which rationalizes the experimental observations and documents the complex network of photochemical and thermal reaction path interconnections. The factors controlling the selection and accessibility of a number of conjugated hydrocarbon prototype conical intersections and ground-state relaxation channels are discussed.