Benzylic cations with triplet ground states: Computational studies of aryl carbenium ions, silylenium ions, nitrenium ions, and oxenium ions substituted with meta π donors

Density functional theory (B3LYP/6-31G(d,p)) was used to predict the effect of meta substitution on aryl cationic (Ar-X⁺) species, including aryloxenium ions, arylsilylenium ions, arylnitrenium ions, and arylcarbenium ions. Multireference second-order perturbation theory (CASPT2) calculations were used to benchmark the quantitative accuracy of the DFT calculations for representative systems. Substituting the meta positions on these species with π donors stabilizes a π,π* diradical state analogous to the wellknown m-xylylene diradical. Notably, the 3,5-bis(N,N-dimethylamino)benzyl cation is predicted to have a triplet ground state by 1.9 kcal/mol by DFT and to have essentially degenerate singlet-triplet states at the CASPT2(10,9) level of theory. Adding electron-withdrawing CF₃ groups to the exocyclic carbon of this meta-disubstituted benzyl cation further increases the predicted singlet-triplet gap in favor of the triplet. Other aryl cationic species substituted with strong π electron-donating groups in the meta positions are predicted to have low-energy or ground-state triplet stales. Systems analogous to the naphthaquinodimethane diradicals are also reported.