HEAT CAPACITY OF BIS(ADIPONITRILE)COPPER(I) NITRATE. AN ORDER-DISORDER TRANSITION IN A DILUTE SYSTEM OF NITRATE IONS

BY MICHAEL JEWESS, SUSAN LEE-BECHTOLD, AND LIONEL A K STAVELEY

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This work was performed in the Inorganic Chemistry Laboratory, University of Oxford, UK, and was published in J. Chem. Soc. Faraday 2.

Abstract

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.

Keywords: thermodynamics, crystallography, disorder, orientational disorder, specific heat, heat capacity, anomaly, adiponitrile, copper(I), nitrate.

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