Heat capacity measurements of chiral and racemic molecular magnets

Thermodynamic nature of the magnetic transitions of a cyano-bridged WV–CuII compound of [W(CN)8]4[Cu(pn)H2O]4[Cu(pn)]2·2.5H2O is studied by low-temperature heat capacity measurements. Distinct thermal anomalies were observed in the temperature dependence of the heat capacity at 7.8 K for chiral compound (S-enantiomer) and 7.2 K for racemic compound. They are associated with the antiferromagnetic transitions reported by the previous magnetization measurements. In the case of S-enantiomer, a shoulder structure of Cp/T has been observed just below the main peak of the antiferromagnetic transition temperature at 0 T. The shoulder grows to a sharp anomaly with a first order character by applying magnetic field of about 1 T in the b-axis direction. On the other hand, just a broadening of the peak was observed for racemic compound, which means that the complicated behavior under magnetic field is derived from the crystallographic chirality. We also studied the behavior of S-enantiomer with magnetic fields applied parallel to the c-axis and found that the first order transition is not induced. Appearance of the complicated peak structure including the first order character specific in the b-direction is ascribed to peculiar magnetic ordering derived from crystallographic chirality related to the Dzyaloshinskii–Moriya (DM) interaction.