Thermodynamic properties and tautomerism of tetrazole

Article abstract of DOI:10.1006/jcht.1993.1156

The results of a study of tetrazole in different phase states are given.The heat capacity of crystalline tetrazole was measured by vacuum adiabatic calorimetry (T = 5 K to 320 K) and by the triple-heat-bridge method (T = 320 K to 400 K).The fusion properties: T_fus = (430 +/- 1) K and Δ_fusH_m deg = (18.0 +/- 0.5) kJ * mol^-1 were obtained by the latter method.The C_p.m = f(T) curve shows a small anomaly at temperatures from 230 to 245 K, connected with a reversible solid-to-solid transition: Δ_trsH_m = (14.0 +/- 1.0) J * mol^-1 and Δ_trsS_m = (0.060 +/-0.004) J * K^-1 * mol^-1.The value of Δ_fG_m deg(cr. 298.15 K) = (363.16 +/- 0.91) kJ * mol^-1 was found on the basis of our own and literature results.The sublimation enthalpy was directly determined by heat-conduction differential microcalorimetry: Δ_subH_m deg(353 K) = (88.16 +/- 0.48) kJ * mol^-1.The saturated vapour pressure of crystalline tetrazole was measured by the integral-effusion-Knudsen method from 333 K to 404 K: ln(p/Pa) = (31.148 +/- 0.458)-(10560 +/- 168)(K/T) and molar enthalpy of sublimation was calculated Δ_subH_m deg(369 K) = (87.80 +/- 1.40) kJ * mol^-1.From these results a weight-averaged molar enthalpy of sublimation Δ_subH_m deg(350 K) = (88.25 +/- 0.45) kJ * mol^-1 was calculated and the conventional molar entropy of gaseous tetrazole S_m deg(g, 350 K) = (274.07 +/- 1.45) J * K^-1 * mol^-1 was found.The ideal-gas entropy of tetrazole was calculated by a statistical-mechanical method from molecular and spectral quantities.The calculated entropy S_m deg(g, 350 K) = 277.52 J * K^-1 * mol^-1 is in satisfactory accordance with the experimental value.So, our thermodynamic investigation shows that tetrazole in the gas phase exists most probably as molecules with a single configuration (without a probable tautomerism 1-H = 2-H).Thermodynamic functions of tetrazole in the solid state (T = 0 to 400 K) and in the ideal-gas state (T = 100 K to 1000 K) were calculated and tabulated.The molar values C_p.m deg, Δ₀^TS_m deg, Δ₀^TH_m deg/T, and Φ_m deg of crystalline and gaseous tetrazole at the temperature 298.15 K are, respectively: (76.52 +/- 0.31) and 52.37; 96.40 +/- 0.41 and 267.12; 45.60 +/- 0.18 and 37.77; and 50.80 +/- 0.45 and 229.35) J * K^-1 * mol^-1.