Anomalous Magnetic Properties of Stable Crystalline Phenoxyl Radicals

Abstract

Abstract The results of static magnetic susceptibility measurements in the 1.8–300°K temperature range are reported on powder samples of the stable aromatic free radicals: 2,6-di-t-butyl-4(3,5-di-t-butyl-4-oxocyclohexa-2,5-dienylidene methyl) phenoxyl (Galvinoxyl) and 2,6-di-t-butyl-4(3,5-di-t-butyl-4-oxocyclohexa-2,5-dienylidene amino) phenoxyl (BIP) radicals. The susceptibility of Galvinoxyl follows the Curie-Weiss law above Tc=81±1°K, depending on the sample, with a Weiss constant of 11±3°K. However, at Tc=81°K, χp rapidly decreases, and below 66°K the susceptibility again increases, with different spin concentrations for independently-prepared samples as the temperature is lowered. A maximum at 81°K was also found by the EPR measurements; the anomaly of the susceptibility at this temperature is probably due to a first-order phase transition induced magnetically, accompanying the pairing of magnetic spins, judging from the results of differential thermal analysis and the magnetic susceptibility measurements. On the other hand, the magnetic susceptibility of the BIP radical exhibits a broad maximum at 54±2°K. This susceptibility can be explained approximately using a model which consists of a singlet ground state and a triplet state lying slightly above the ground state.