Monoaza-analogs † of trimethylenemethane. Isoelectronic similarities and differences

Calculations employing multireference second-order perturbation theory and density functional theory have been carried out for a series of monoaza-analogs of trimethylenemethane (TMM). Trimethyleneammonium is predicted to have multiplet splittings very similar to TMM. Iminoallyl, also isoelectronic with TMM but with heteroatomic perturbation at a terminal position, is a ground-state triplet like TMM but its corresponding closed-shell singlet state is stabilized relative to the triplet by about 7 kcal mol^-1. Protonation of iminoallyl generates iminiumdimethylenemethane and the latter system has a singlet ground state in violation of Ovchinnikov's rule. Aqueous medium effects on the singlet and triplet states of iminiumdimethylenemethane are calculated to reduce the singlet-triplet splitting by 1.6 kcal mol^-1 using Solvation Model 5.4/AM1.