Spin density in a triazole-nitronyl-nitroxide radical presenting linear ferromagnetic interactions: role of hydrogen bonding

The compound 2-{3-[4-methyl-1,2,4-triazolyl]}-4,4,5,5,-tetramethylimidazoli ne-1-oxyl-3-oxide, abbreviated as Metrz-Nit, crystallizes in the non-centro symmetric space group P2(1)2(1)2(1). The investigation of the magnetic prop erties has revealed the occurrence of intermolecular ferromagnetic interact ions. The crystal structure has been refined by neutron diffraction at 11 K . The spin density distribution has been determined from polarized neutron diffraction experiments carried out at 1.5 K under a magnetic field of 5 T. As expected, the main contributions of the spin distribution have been obs erved in the 2p(pi) orbitals of the nitrogen and oxygen atoms of the two NO groups, and a significant negative spin population has been detected on th e sp(2) carbon atom of the nitronyl nitroxide moiety. The spin distribution is slightly dissymmetrical, so that the sp(3) carbon atoms in alpha-positi on of the nitro nitrogen atoms carry spin populations of opposite signs. Co ncerning the triazole ring, the main spin population, of negative sign, has been found on the nitrogen atom occupying the 3-position. The carbon atom of the methyl group attached to the 4-position has been also found to carry a significant negative spin population. The spin populations on the hydrog en atoms have been determined. These experimental data have been compared t o the results of LSD calculations performed on an isolated molecule. The ro le of intermolecular interactions in the spin distribution has been discuss ed. (C) 1999 Elsevier Science B.V. All rights reserved.