The mechanism of the photochemical hydrogen migration in 1,3,5-cycloheptatriene: A theoretical study

Based on CASSCF calculations for the reaction profile of the photochemical [1,7]-sigmatropic hydrogen shift in 1,3,5-cycloheptatriene (CNT), the detai led mechanism of the excited-state reaction is discussed. The results are i n agreement with the observed ultrafast rates for passage from the initiall y-excited 1A" state by way of the dark 2A' state to the ground-state (1A') potential-energy surface. The kinetics and the selectivity of the reaction are characterized by a small energy barrier on the dark 2A' state that sepa rates the excited-state minimum from a conical intersection at pericyclic g eometries. This intersection is responsible for efficient, excited-state de activation. At the CASSCF level the barrier height is calculated to be 7.3 kcal mol(-1). If dynamic correlation effects are taken into account with th e CASPT2 method, the barrier is reduced to 3.8 kcal mol(-1) The existence o f an excited-state barrier as the decisive mechanistic feature of the hydro gen migration was verified by considering 1-substituted CHT. The barriers c alculated for sigmatropic shifts away from and toward the substituent diffe r considerably In agreement with experimental observation, the smaller barr ier is found for migration away from an acceptor and toward a donor substit uent. The differences in barrier height are in excellent agreement with the experimentally observed product distribution.