Intramolecular electron transfer in bis(methylene) adamantyl radical cation: a case study of diabatic trapping

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 phenomenon have been derived. The surface is centered around a conical intersection, and diabatic trapping occurs because 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 of the rigid carbon framework. Other modes (including methylene torsions and pyramidalizations) may lift the degeneracy at second-order but do not affect the energy gap at the intersection region effectively. The resulting topology is similar to that of an (n - 1) dimensional seam (where n is the number of nuclear degrees of freedom of the molecule) that cannot be avoided along the reaction coordinate, thus favoring recrossing to the upper surface. This analysis is extended by ab initio semiclassical dynamics using an Ehrenfest and a trajectory surface hopping algorithm implemented at the CASSCF level. Examination of the trajectories shows that there is no single mode that controls the diabatic trap, in agreement with the condition that there is no predominant degeneracy-lifting coordinate. Thus the reactivity depends on a combination of small effects, where presumably higher-order effects come into play. This should be the general behavior of dynamics at a diabatic trapping situation.