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.

With the purpose of exploring the reliability of the enthalpies of formation calculated using G2 methods, we have examined a series of saturated and unsaturated alicyclic hydrocarbons varying the size and the number of formal double bonds in the molecule. Heats of formation have been calculated at the G2(MP2) and G2 levels through both atomization reactions and bond separation isodesmic reactions, and comparison with experimental values has been made. A linear relationship between the differences between experimental and calculated (from atomization reactions) heats of formation and the number of formal double bonds is obtained.

With the purpose of exploring the reliability of the enthalpies of formation calculated using the G3 method, we have examined a series of saturated and unsaturated alicyclic hydrocarbons varying the size and the number of formal double bonds in the molecule. Heats of formation have been calculated at the G3 level through both atomization reactions and bond separation isodesmic reactions, and comparisons with experimental values and with values previously calculated at the G2(MP2) and G2 levels have been made. The quality of the G3-calculated enthalpies of formation using atomization reactions is comparable to that obtained at the G2 level using bond separation reactions, whereas G3 calculations are two to three times faster than G2 calculations.