Thermal behaviour and thermochemistry of hexachlorozirconates of mononitrogen aromatic bases

Abstract

Thermal features of the hexachlorozirconates of the base (designated B) pyridine, quinoline (isoquinoline) and acridine, as well as several of their methyl substituted derivatives were examined by dynamic and quasi-isothermal-isobaric thermoanalytical methods. These measurements enabled determination of the temperatures of phase transitions, melting and the onset of decomposition. All the compounds undergo dissociation upon increase of temperature, leading to their partial volatilization. It is believed that the primary process, which can be summarized with the equation (BH) 2ZrCl 6(c) → 2B(g) + 2HCl(g) + ZrCl 4(ads) is accomplished in several stages, attaining the release of complementary ions from the lattice, formation with these of an intermediate conglomerate loosely bound to the crystal surface and subsequent dissociation of the latter into base, HCl and ZrCl 4. Interaction between primarily released molecules was examined at the level of the STO-3G ab inito method considering all electrons and the semiempirical AM1 and PM3 quantum chemistry methods. Decomposition of the compounds is accompanied by side processes affording ZrO 2 and sometimes carbonization products of the organic constituents. The application of the van't Hoff equation to the non-isothermal thermogravimetric curves enabled evaluation of the enthalpies and temperatures of completion of the thermal dissociation. Using values of the former quantity and other thermochemical characteristics available in the literature, the enthalpies of formation and the crystal lattice energies of the compounds were evaluated. The values of the latter quantity were further estimated following the Kapustinskii-Yatsimirskii approach. We tried to reveal relationships between structural, energetic and physicochemical features of the neutral and protonated organic bases, derived by the semiempirical AM1 and PM3 quantum chemistry methods. Furthermore, an attempt was made to correlate the thermal behaviour and thermochemical characteristics of hexachlorozirconates with features of these neutral and protonated base molecules.