THERMAL-BEHAVIOR AND THERMOCHEMISTRY OF HEXACHLOROZIRCONATES OF MONONITROGEN AROMATIC BASES

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 q uasi-isothermal-isobaric thermoanalytical methods. These measurements enabled determination of the temperatures of phase transitions, meltin g and the onset of decomposition. All the compounds undergo dissociati on upon increase of temperature, leading to their partial volatilizati on. It is believed that the primary process, which can be summarized w ith the equation (BH)2ZrCl6(c) --> 2B(g) + 2HCl(g) + ZrCl4(ads) is acc omplished in several stages, attaining the release of complementary io ns from the lattice, formation with these of an intermediate conglomer ate loosely bound to the crystal surface and subsequent dissociation o f the latter into base, HCI and ZrCl4. Interaction between primarily r eleased molecules was examined at the level of the STO-3G ab inito met hod considering all electrons and the semiempirical AM1 and PM3 quantu m chemistry methods. Decomposition of the compounds is accompanied by side processes affording ZrO2 and sometimes carbonization products of the organic constituents. The application of the van't Hoff equation t o the non-isothermal thermogravimetric curves enabled evaluation of th e enthalpies and temperatures of completion of the thermal dissociatio n. Using values of the former quantity and other thermochemical charac teristics available in the literature, the enthalpies of formation and the crystal lattice energies of the compounds were evaluated. The val ues of the latter quantity were further estimated following the Kapust inskii-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 qu antum chemistry methods. Furthermore, an attempt was made to correlate the thermal behaviour and thermochemical characteristics of hexachlor ozirconates with features of these neutral and protonated base molecul es.