Crystal structure and thermochemistry of anhydrous potassium 2-pyrazine carboxylate (C5H3KN2O2)

Potassium 2-pyrazine carboxylate C5H3KN2O2 [abbreviated as K(PCA)(s)] was synthesized, and the synthesis was carried out in the mixed solvent of ethanol and water. Its crystal structure was characterized by X-ray crystallography. And its low-temperature heat capacities were measured by use of a precise automatic adiabatic calorimeter with a small sample in the temperature region of 78–400 K. A polynomial equation of experimental molar heat capacities as a function of the temperature has been obtained. The smoothed molar heat capacities and thermodynamic functions of the compound were calculated based on the fitted polynomial equation. A reasonable thermochemical cycle was designed on the basis of the synthesis reaction of the substance, and the standard molar enthalpies of dissolution for the reactants and products of the reaction in the selected solvent were measured by an isoperibol solution-reaction calorimeter, respectively. The enthalpy change of the reaction was calculated as—(65.396 ± 0.388) kJ mol−1. Finally, the standard molar enthalpy of formation of the title complex was calculated as—(532.106 ± 1.196) kJ mol−1 by using the Hess thermochemical cycle. Furthermore, the reliability of the designed thermochemical cycle was verified by UV–Vis spectra of the solutions obtained from the dissolution of reactants and products.