Cyclooctatetraene computational photo- and thermal chemistry: A reactivity model for conjugated hydrocarbons

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 _8h-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.