Thermodynamic properties and tautomerism of tetrazole
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 fus H o m = (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 trs H m = (14.0+/-1.0) J.mol -1 and Δ trs S m = (0.060+/-0.004) J.K -1 .mol -1 . The value of Δ f G o m (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 sub H o m (353 K) = (88.16+/-0.48) kJ. -1 . The saturated vapour pressure of crystalline tetrazole was measured by the integral-effusion-Knudsen method from 333 K to 404 K: In(p/Pa) = (31.148+/-0.458)-(10560+/-168)(K/T) and molar enthalpy of sublimation was calculated Δ sub H o m (369 K) = (87.80+/-1.40) kJ.mol -1 . From these results a weight-averaged molar enthalpy of sublimation Δ sub H o m (350 K) = (88.25+/-0.45) kJ.mol -1 was calculated and the conventional molar entropy of gaseous tetrazole S o m (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 spectral quantities. The calculated entropy S o m (g, 350 K) = 277.52 J.K -1 is in satisfactory accordance with the experimental value. So, our thermodynamic investigation shows that tetrazole in the gas phase exist 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 o p. m , Δ T O S o m , Δ T O H o m /T, and Φ o m 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 .