Studies on a "Disappearing Polymorph": Thermal and Magnetic Characterization of α-p-NCC₆F₄CNSSN^•

The α-and β-phases of the thiazyl radical p-NCC₆F₄CNSSN^• (1) can be selectively prepared by careful control of the sublimation conditions, with the α-phase crystallizing preferentially when the substrate temperature is maintained below -10 °C, whereas the β-phase is isolated when the substrate temperature is maintained at or above ambient temperature. Differential scanning calorimatry studies reveal that the α-phase converts to the β-phase upon warming over the range 111-117 °C (ΔH = +4 kJ·mol^-1) via a melt-recrystallization process, with the β-phase itself melting at 167-170 °C (ΔH_fus = 27 kJ·mol^-1). IR and Raman spectroscopy can be used to clearly discriminate between 1α and 1β. The α-phase shows a broad maximum in the magnetic susceptibility around 8 K that, coupled with a broad maximum in the heat capacity, is indicative of short-range order. Some field dependence of the susceptibility below 3 K is observed, but the lack of features in the ac susceptibility, M vs H plots, or heat capacity mitigates against long-range order in 1α.