Solid State 1H-Mas-Nmr and Spin Densities on Protons of the Organic Ferromagnetic Tempo Derivatives

Electron spin densities on hydrogen atoms of 4-(arylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyls (abbreviated as Ar-CH=N-TEMPO), which show ferromagnetic behavior at low temperatures, were determined in their crystal phases from the temperature dependence of the Fermi contact shift measured by high speed magic angle spinning proton nuclear magnetic resonance. This method revealed a large negative hyperfine coupling constant for the methyl and methylene protons, A = -1.00 MHz for Ar = p-C1-ph and A = -1.32 MHz for Ar = ph, and very small one for the aryl group protons, |A| < 0.01 MHz for p-Cl-ph and A = +0.04 MHz for ph. The observed negative hyperfine coupling constant (negative spin density) of methyl and methylene protons matches with spin alternation for the intermolecular spin polarization mechanism through the contact of methyl and/or methylene protons to adjacent N-O radical group. This contact potentially contributes to the intermolecular ferromagnetic interaction.