Nucleophilic Attack on Cation Radicals and Cations. A Theoretical Analysis

The curve-crossing model is applied to the problem of barrier heights for nucleophilic attack on cation radicals. RH^•+, and cations, R⁺. It is shown that the barrier height depends on the ionization potential of the nucleophile, the electron affinity of the cation, and, for cation radicals, also on the singlet-triplet energy gap of the corresponding neutral molecule, RH. It is shown that in general, cation radicals are likely to be less reactive than cations (of the same acceptor ability) toward nucleophilic attack, because the product configuration for cation radicals is doubly excited (D⁺³*A^-), whereas that for regular cations is singly excited (D⁺A^-). A semiquantitative analysis is presented that shows that those cases where cation radicals are likely to react rapidly with nucleophiles can be predicted in a straightforward manner.