Transition state ring sizes in H-transfers in closed shell aliphatic cations

We use theory to determine the relative rates of 4-, 5- and 6-membered ring H-transfers in closed shell cations, a comparison not possible experimentally due to interference from competing processes. H-transfers of increasing ring size were characterized by ab initio, density functional and/or RRKM studies of CH3N+HCH2, NH2CHN+HCH3, NH2CH2N+HCH2, NH2CH2CH2N+HCH2, N+H2CHCH2NHCH3 and CH3NHCH2NH+CH2. The critical energies for these reactions decreased and the rates thereof increased dramatically with increasing H-transfer ring size. Transition state ring strain and avoiding orbital symmetry constraints together produce the same sequence of reaction rates (6 > 5 > 4 membered ring preference) and critical energies (4 > 5 > 6 membered ring order) in open and closed shell organic cations. Twisting around double bonds during H-transfers breaks conjugation and allows the reactions to bypass orbital symmetry constraints.