The photoinduced cis−trans isomerization of all-trans retinal protonated Schiff base is studied using the full multiple spawning method. Our model allows for two classes of electronic transitions:  exciton migration and quenching by coupling of the electronic and nuclear degrees of freedom. We find that quenching and exciton transfer are two temporally disjoint events and that exciton transport is highly directed. The observed selectivity in photoproducts is interpreted in terms of an electronic energy “funnel”, which is attributed to the Schiff base. The limits of the concept of an electronic energy “funnel”, when electron−phonon coupling is strong, are discussed and it is argued that such funnels can induce selectivity in excitation migration even when thermodynamic equilibrium among the excited electronic states is not reached.

The mechanism of thermal deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) is examined both theoretically and experimentally in this paper. The theoretical work involves CASPT2 ab initio MO calculations using the 6-31G(d) basis set. These calculations are shown to give relative enthalpies for the stationary points on the potential energy surface that are within ±3 kcal/mol of the experimental values. Surprisingly, the transition structure for the rate-limiting step is found to correspond to synchronous C−N cleavage, leading to the formation of the cyclopentane-1,3-diyl biradical. The alternative mechanism, involving stepwise C−N bond scission, is found to have an activation enthalpy that is at least 7 kcal/mol higher. The synchronous mechanism is in apparent conflict with earlier calculations and experiments, all of which seemed to favor the stepwise pathway. A resolution of this disagreement is proposed. The well-known preference for inversion of stereochemistry in the deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene-exo,exo-5,6-d 2 (DBH-d 2 ) is proposed to involve nonstatistical dynamical effects. This mechanism is similar to, but not identical with, one first proposed by Allred and Smith over 30 years ago. The deazetization of DBH-d 2 in supercritical propane is studied experimentally. The pressure dependence of the reaction stereochemistry is shown to be qualitatively and quantitatively consistent with the dynamic model, but not to be consistent with any mechanisms that would require separate reaction paths across the potential energy surface. In supercritical carbon dioxide, the pressure dependence is found not to be fit by any simple model. Calculations suggest that this may be because CO2 forms weak complexes (binding energies of approximately 2 kcal/mol for each CO2) with the cyclopentane-1,3-diyl intermediate. The calculations also suggest that similar complexes can be expected between CO2 and simple monoradicals.

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