Theoretical study of the effective exchange interactions between nitroxides via hydrogen atoms

Effective exchange interactions between nitroxides via hydrogen atoms are studied for several model clusters which involve bridge hydrogen atoms at idealized positions. Ab initio post-UHF and UNO CASCI calculations indicate two different mechanisms for ferromagnetic interactions: (1) the spin polarization (SP) effect via a bridge hydrogen and (2) the bridge hydrogen hyperconjugative effect based on the π∗-like hyperconjugated SOMO. Contributions of the kinetic, potential and SP terms are estimated by UNO CASCI {2,2} and APUHF, and the electron correlation term by MP and CCSD(T). Bridge hydrogen atoms are shown to be important for weak effective exchange interactions between nitroxides. The implications of these findings are discussed in relation to the molecular design of crystalline organic ferromagnets.