Phase transition analysis of 5-aminotetrazole from room temperature to the melting point.

The phase transition behavior of the 5-aminotetrazole (5-ATZ) crystal was studied in the range of room temperature to the melting point by the differential scanning calorimetry and powder X-ray diffraction techniques to investigate the dehydration phenomenon and to obtain the physical state of the high temperature phase. We found that 5-ATZ has one monohydrate crystal phase and three anhydrous crystal phases (I, II, and III) up to its melting point. The dehydration reaction of the monohydrate crystal occurred at about 50 °C. The dehydration enthalpy was 12.2 kcal/mol. The required activation energy was calculated using the Kissinger method, yielding the value of 27.1 kcal/mol. The phase transitions accompanying the sublimation from phase I to phase II and from phase II to phase III were observed during the prolonged heating at 175 °C. The dehydration phenomenon was also examined through quantum chemical calculations, and the calculated energy to eliminate a water molecule from the monohydrate crystal by the density functional theory at the M06/cc-pVTZ level basically agreed with the experimentally determined dehydration enthalpy.