Crystal structure and thermochemical properties of 2-pyrazine carboxylate lithium monohydrate [Li(pyza)(H2O)] n (s)(pyza=2-pyrazine carboxylate)

Abstract

A 2-pyrazine carboxylate lithium monohydrate [Li(pyza)(H2O)] n was synthesized in a mixed solution of redistilled water and anhydrous ethanol. X-Ray crystallography was applied to characterizing its crystal structure. Low temperature molar heat capacities were measured in a temperature range of from 78 K to 400 K with a precision automatic adiabatic calorimeter. Two polynomial equations of experimental molar heat capacity as a function of temperature were obtained by the least-squares method. The smoothed molar heat capacities and thermodynamic functions of the compound were calculated based on the fitted polynomial equations. In accordance with Hess’s law, a reasonable thermochemical cycle was designed based on the preparation reaction of the target compound. The standard molar enthalpies of dissolution for the reactants and products of the designed thermochemical reaction were measured by an isoperibol solution-reaction calorimeter, and the enthalpy change of the reaction was obtained, i.e., Δr H m θ =-(30.084±0.329) kJ/mol. The standard molar enthalpy of the formation of the target compound was determined as $\Delta _f H_{m\{ [Li(pyza)(H_2 O)]_n (s)\} }^\theta = - (260.844 \pm 1.178)$ kJ/mol based on the enthalpy change of the reaction and standard molar enthalpies of the formation of other reactants and products. In addition, UV-Vis spectroscopy and the data of the refractive indexes were used to confirm whether the designed Hess thermochemical cycle was reasonable and reliable.