Thermal features and thermochemistry of hexachlorozirconates of aliphatic and aromatic mono-amines—stability of hexahalogenozirconates

Abstract

Thermal behaviour of the hexachlorozirconates of several alkanamines and aromatic mono-amines was examined using dynamic and quasi-isothermal-isobaric thermoanalytical methods. Decomposition of the compounds upon an increase in temperature is accompanied by partial volatilization. The residue contains ZrO 2 and is sometimes contaminated with traces of carbonization products. It is believed that the primary process, which can be summarized with the equation (where A denotes an alkyl or aryl substituent ( p=1–4; a=0 and s=1 for quaternary, and a=1 and s=0 for other compounds studied)) is followed by instantaneous oxidation of zirconium tetrachloride remaining in the condensed phase (cond). An insight into the thermodynamics of the compounds became possible on employing the van't Hoff equation to the non-isothermal thermogravimetric curves. This enabled evaluation of the enthalpies of the thermal decomposition and consequently the enthalpies of formation and the crystal lattice energies of the salts. The latter quantity was further examined using the Kapustinskii-Yatsimirskii method. Geometries, energies and other physicochemical properties of simple aliphatic and aromatic amines and their protonated forms were determined by AM1 and PM3 methods in order to reveal which of these correlate with the proton affinity of amines and the thermal behaviour and thermochemical characteristics of hexachlorozirconates. In addition, the influence of dimensions of ions on the thermodynamic stability of hexahalogenozirconates, with respect to dissociation and oxidation processes, was studied.