FERROMAGNETIC AND ANTIFERROMAGNETIC INTERMOLECULAR INTERACTIONS IN THE AROMATIC HETEROCYCLIC-DERIVATIVES OF ALPHA-NITRONYL NITROXIDE

Magnetization isotherms up to 5.5 T and temperature dependence of magnetic susceptibility down to 1.8 K of the organic neutral radicals, 3- and 4-quinolyl nitronyl nitroxide and 2,6-pyridincdiyl bis(nitronyl nitroxide), are described. 3-quinolyl nitronyl nitroxide exhibit ferromagnetic intermolecular interaction with the Weiss constant THETA = 0.27 K, whereas 4-quinolyl nitronyl nitroxide shows antiferromagnetic intermolecular interaction with the exchange coupling constant J/k = -7.8 K. This significant difference between the magnetic properties of two compounds are discussed in terms of their crystal structures. The temperature dependence of product of paramagnetic susceptibility and temperature, chi(p)T, of a biradical 2,6-pyridinediyl bis(nitronyl nitroxide) shows a plateau at low temperatures in addition to the usual plateau observed in the high-temperature region. This peculiar behavior is interpreted in terms of the multi-spin system associated with a combined effect of ferro- and antiferro-magnetic couplings.