No-overlap and orientation principle for ferromagnetic interactions between nitroxide groups

Approximately spin-projected UMP4(2) and UCC SD(T) computations show that ferromagnetic intermolecular interactions are feasible for T-shape and rhombus conformations in parallel interplane orientations for nitroxide pairs because of no-overlap between π∗-type SOMOs (no kinetic exchange) and non-zero potential exchange via SOMO-SOMO through-space contact. On the other hand, antiferromagnetic kinetic exchange interactions via SOMO-SOMO overlap are predominant for rectangular conformations. The no-overlap and orientation principle proposed previously is a useful guide for the theoretical understanding of ferromagnetic exchange interactions between nitroxide groups. Implications of the calculated results are discussed in relation to the discovery of ferromagnetic interactions in the crystals of adamantane dinitroxide, TEMPO derivatives and MOTMP.