Theoretical study of the 1,2 rearrangement of housane radical cations:: Key role of a transient cyclopentane-1,3-diyl intermediate

CASSCF computations suggest that the ground-state potential energy surface of housane radical cations is centered around a conical intersection (and its surrounding Jahn-Teller-type surface) at a planar, symmetric cyclopentane-1,3-diyl geometry, In our reactivity model, this region is connected to the reactants via a bridge-bending coordinate and to the products via a shift coordinate. The preference for the spin-localized planar intermediate is caused by the preferential energy stabilization along a charge localization coordinate (the derivative-coupling coordinate at the conical intersection). Mechanistically, our computations show that the reaction proceeds in two steps: the breakage of the one-electron bond of the reactants, which produces the asymmetric, quasi-planar intermediate and is the rate-determining step, and the subsequent 1,2 rearrangement, which is essentially barrierless. The reaction results in the selective 1,2 migration of the original endo substituent of the reactant.