| Abstract
| - The structure of the potential energy surface for the intramolecular electron transfer (IET) of fourdifferent model radical cations has been determined by using reaction path mapping and conical intersectionoptimization at the ab initio CASSCF level of theory. We show that, remarkably, the calculated paths residein regions of the ground-state energy surface whose structure can be understood in terms of the position andproperties of a surface crossing between the ground and the first excited state of the reactant. Thus, in thenorbornadiene radical cation and in an analogue compound formed by two cyclopentene units linked by anorbornyl bridge, IET proceeds along direct-overlap and super-exchange concerted paths, respectively, thatare located far from a sloped conical intersection point and in a region where the excited-state and ground-state surfaces are well separated. A second potential energy surface structure has been documented for 1,2-diamino ethane radical cation and features two parallel concerted (direct) and stepwise (chemical) paths. Inthis case a peaked conical intersection is located between the two paths. Finally, a third type of energy surfaceis documented for the bismethyleneadamantane radical cation and occurs when there is, effectively, a seam ofintersection points (not a conical intersection) which separates the reactant and product regions. Since thereaction path cannot avoid the intersection, IET can only occur nonadiabatically. These IET paths indicatethat quite different IET mechanisms may operate in radical cations, revealing an unexpectedly enriched andflexible mechanistic spectrum. We show that the origin of each path can be analyzed and understood in termsof the one-dimensional Marcus−Hush model.
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