Heat capacity of bis(adiponitrile)copper(I) nitrate. An order–disorder transition in a dilute system of nitrate ions
Heat capacity measurements were made on bis(adiponitrile)copper(I) nitrate, Cu[NC(CH2)4CN]2NO3, covering the temperature range 1.5–307 K. There is an anomalous heat capacity contribution over the range ≈ 37 to ≈ 70 K, with heat capacity maxima at ≈ 50 and ≈ 63 K. The total anomalous entropy gain was estimated as 0.8 R ln 2, and we accordingly suggest that it corresponds to the conversion of an ordered arrangement of nitrate ions into one in which each nitrate ion can take up each of two positions with probability ½. The double maximum can possibly be attributed to the complex spatial distribution of nitrate ions in the crystal. The temperature of the anomaly is much lower than the temperature at which nitrate ions first become disordered in simple nitrates such as sodium nitrate, and this is ascribed to the much greater dilution of the nitrate ions.
The room-temperature (and presumably low-temperature) structure of the compound is extraordinary in that it contains six separate three-dimensional interpenetrating cationic networks, but we have not found it necessary to ascribe any feature of the heat capacity results to this.