Low-temperature heat-capacity and standard molar enthalpy of formation of the coordination compound Zn(Phe)3(NO3)2 • H2O(s) [Phe = L-α-phenylalanine]

Low-temperature heat capacities of the coordination compound Zn(Phe)(3)(NO3)(2) . H2O(s) have been precisely measured over the temperature range between T = 78 K and T = 373 K through a small sample precision automated adiabatic calorimeter. Experimental heat capacities have been fitted into a polynomial equation of heat capacities (C-p,C-m) with the reduced temperatures (x), [x = f(T)]. The initial dehydration temperature of the complex (T-d) has been given from analysis of the heat-capacity curve, T-d = 327.25 K. Dissolution enthalpies of [ZnSO4 . 7H(2)O(s) + 2NaNO(3)(s) + 3L-Phe(s)] (Delta(d)H(m,l)(0)) and [Zn(Phe)(3)(NO3)(2) . H2O(s) + Na2SO4(s)] (Delta(d)H(m,2)(0)) in 100 mL of 2 molar HCl at T = 298.15 K have been measured by means of an isoperibol solution calorimeter: Delta(d)H(m,l)(0) = (115.430 +/- 2.797)kJ . mol(-1), Delta(d)H(m,2)(0) = (67.211 +/- 2.147)kJ . mol(-1). The standard molar enthalpy of formation of the complex has been determined to be: Delta(f)H(m)(0) = -(2292.44 +/- 11.12)kJ . mol(-1), through a Hess thermochemical cycle from the results of molar dissolution enthalpies and other auxiliary thermodynamic data. (C) 2003 Elsevier Science Ltd. All rights reserved.