| Research Abstract |
Magnetic properties of substances have been interpreted on the basis of either of two models : one is the itinerant-electron model for delocalized-electron systems and the other is the Heisenberg model for localized-electron systems. Organic free-radicals and charge-transfer complexes, in which the unpaired electron is not localized on a particular atom but delocalized over many atoms in a molecule to a great extent, belong to an intermediate category between the two models. Magnetic properties of these radicals have so far been described as a perturbation from the two extreme models. Main purpose of the present research project was to elucidate magnetic properties of organic free-radicals and charge-transfer complexes on the basis of precise heat capacity and magnetic susceptibility measurements at low temperatures.
Compounds for which heat capacity measurements were made by adiabatic calorimeters involve (1) charge-transfer molecular ferromagnets, decamethylferrocenium-TCNE and decame
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thylferrocenium-TCNQ, (2) pure organic free-radicals, MOTMP and AOTMP, (3) charge-transfer complex TTF-CA that shows the ionicneutral phase transition, (4) binuclear mixed-valence complex, diiodobiferrocenium triiodide, and (5) mixed-metal coordination compounds, MnCu (obbz) H_2O, MnCu (obbz) 5H_2O, and NBu_4[CuCr (Ox) _3].
All the magnetic compounds exhibited a phase transition peak arising from magnetic ordering and a heat capacity anomaly due to the short-range order effect. By applying the spin-wave theory to temperature dependence of the magnetic heat capacities at low temperatures, the dimensionality of the spin-lattice and the nature of the ordered spin state were determined. The spin-spin interaction in the radical complexes has been proved to be described by the Heisenberg model, in which only the exchange integral parameter is effectively modified. Observation of a Schottky anomaly in the mixed-valence complex, 1', l"'-diiodobiferrocenium triiodide, predicts that the intramolecular electron transfer occurs by a tunnel-mechanism. Less
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