title: Cyclooctatetraene computational photo- and thermal chemistry: a reactivity model for conjugated hydrocarbons.
abstract: 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 (S(1)) takes place after ultrafast decay from the spectroscopic excited state (S(2)). A planar D(8)(h)-symmetric minimum represents the collecting point on S(1). Nonadiabatic transitions to S(0) appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S(1) 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.
authors.affiliation: Dipartimento di Chimica G. Ciamician, Università di Bologna, Via Selmi 2, 40126 Bologna, Italy. mgara@ciam.unibo.it
authors.forename: Marco
authors.initials: M
authors.lastname: Garavelli
authors.forename: Fernando
authors.initials: F
authors.lastname: Bernardi
authors.forename: Alessandro
authors.initials: A
authors.lastname: Cembran
authors.forename: Obis
authors.initials: O
authors.lastname: Castaño
authors.forename: Luis Manuel
authors.initials: LM
authors.lastname: Frutos
authors.forename: Manuela
authors.initials: M
authors.lastname: Merchán
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci