HEAT CAPACITIES OF β-MODIFICATION PHTHALOCYANINES FROM

4 K TO 45 K. ANOMALOUS BEHAVIOUR IN COPPER(II) AND NICKEL(II) PHTHALOCYANINE

BY ROBERT M CLAY, MICHAEL JEWESS, 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 capacities from ca. 4 to ca. 45 K are reported for β-modification metal-free, copper(II) and nickel(II) phthalocyanine (β-H₂Pc, β-CuPc and β-NiPc where Pc = C₃₂H₁₆N₈). β-CuPc undergoes two essentially first-order transitions implying the existence of three distinct phases, phase III stable below ca. 9 K, phase II stable from ca. 9 to ca. 12 K and phase I stable above ca. 12 K. Moreover, the sum of the two entropies of transition (for III II and II I) is so large (ca. R ln 4, possibly more) that it is very likely that phase I is disordered in some way. To explain the value R ln 4, we tentatively propose (i) that the disorder in phase I is positional disorder of the Cu²⁺ ions, to each of which there are presumed to be available four nearly equivalent potential minima in the central area of the Pc^2– ligand, and (ii) that in phase III the Cu²⁺ ions are ordered under the influence of their mutual interaction.At temperatures between 4 and 8 K, the molar heat capacity of β-NiPc considerably exceeds those of β-H₂Pc and β-CuPc, and the latter heat capacities, unlike the former, are proportional to T³. This suggests that the heat capacity of β-NiPc below 8 K differs from those of β-H₂Pc and β-CuPc in including a substantial non-vibrational contribution.

Keywords: thermodynamics, crystallography, disorder, positional disorder, specific heat, heat capacity, anomaly, phthalocyanine, copper(II).

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