Thermochemistry On The Coordination Behavior Of 2-pyrazinecarboxylic Acid With Metallic Elements.

2-Pyrazinecarboxylic acid is a kind of important N-heterocyclic ligand. Based on the wide range of physical properties and structural diversities, its metal complexes are important organic-inorganic hybrid materials, which has aroused great interest of chemical workers and become one of the hot spots in relative research areas. In addition, these complexes exhibit attractive application prospects in the development of new functional materials, which include selective catalysis, molecular recognition, electro-optic materials, semiconductor materials, magnetic materials, and so on.Great progress has been made for synthesis and characterization of the structures and properties about the complexes of 2-pyrazinecarboxylic acid with relevant metal salts. However, thermodynamic data for these complexes are needed in order to improve the technique of chemical synthesis in which they participate and make them more suitable for industrial production, develop their new application fields, and carry out relevant theoretical research. In this thesis, our main results for measurements and researches are as follows:1. Ten kinds of the complexes of 2-pyrazinecarboxylic acid with some metal salts were synthesized. The composition, crystal structure, and possible mechanisms of the thermal decomposition of these complexes were determined by elemental analysis, X-ray crystallography, thermogravimetry, and differential scanning calorimetry.2. Experimental molar heat capacities of these complexes were measured in the temperature range from 78 to 400 K by a fully automated precision adiabatic calorimeter. The adiabatic calorimetric experiments have revealed that the structures of most complexes were stable in the temperature range of 78 to 400 K, only several hydrated compounds displayed different extent of dehydration under the temperature of 400 K, which was in agreement with the results of TG analyses. Valid and virtual section of the heat-capacity curve of the compound itself was fitted by means of the least square method and a polynomial equation of experimental molar heat capacities (C_p,m) vs. reduced temperature(X), X = f(T), has been obtained. The smoothed molar heat capacities and thermodynamic functions [H_T-H_298.15K], [S_T-S_298.15K], and [G_T-G_298.15K] relative to the standard reference temperature _298.15K of these complexes were calculated based on the fitted polynomial equation of the heat capacities and tabulated with the interval of 5 K.3. In accordance with Hess's law, a reasonable thermochemical cycle was designed based on the preparation reaction of every target complex. 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. The standard molar enthalpy of formation of the target complex was determined based on the enthalpy change of the reaction and standard molar enthalpies of 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.4. Analytical investigation on the change tendency of heat capacities, thermodynamic functions, and standard molar enthalpies of formation of the 2-pyrazinecarboxylate complexes with their molecular weights has been carried out. Results showed that regular changes displayed between the thermodynamic data and molecule weights of the complexes which have the same coordination mode and spatial configuration.