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| - Intramolecular Electron Transfer in Bis(methylene) AdamantylRadical Cation: A Case Study of Diabatic Trapping
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| - Our characterization of the potential energy surface for electron transfer (ET) in the bis(methylene)adamantane (BMA) model radical cation shows that the surface topology is prone to diabatic trapping(competition between ET and upward hops to the excited state). The general conditions for this phenomenonhave been derived. The surface is centered around a conical intersection, and diabatic trapping occursbecause one of the branching space coordinates (coordinates that lift the degeneracy at first order)corresponds to a vector of small length. For BMA, this coordinate is an antisymmetric breathing mode ofthe rigid carbon framework. Other modes (including methylene torsions and pyramidalizations) may lift thedegeneracy at second-order but do not affect the energy gap at the intersection region effectively. Theresulting topology is similar to that of an (n − 1) dimensional seam (where n is the number of nucleardegrees of freedom of the molecule) that cannot be avoided along the reaction coordinate, thus favoringrecrossing to the upper surface. This analysis is extended by ab initio semiclassical dynamics using anEhrenfest and a trajectory surface hopping algorithm implemented at the CASSCF level. Examination ofthe trajectories shows that there is no single mode that controls the diabatic trap, in agreement with thecondition that there is no predominant degeneracy-lifting coordinate. Thus the reactivity depends on acombination of small effects, where presumably higher-order effects come into play. This should be thegeneral behavior of dynamics at a diabatic trapping situation.
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