Enthalpies of Formation of N -Substituted Pyrazoles and Imidazoles

Accurate experimental enthalpies of formation measured using static bomb combustion calorimetry, the “vacuum sublimation” drop calorimetry method, and the Knudsen-effusion method are reported for the first time for four azoles: 1-methylimidazole ( 1MeIMI ), 1-methylpyrazole ( 1MePYR ), 1-benzylimidazole ( 1BnIMI ), and 1-benzylpyrazole ( 1BnPYR ). These values and those corresponding to imidazole ( 1HIMI ), pyrazole ( 1HPYR ), 1-ethylimidazole ( 1EtIMI ), 1-ethylpyrazole ( 1EtPYR ), 1-phenylimidazole ( 1PhIMI ), and 1-phenylpyrazole ( 1PhPYR ) are compared with theoretical values using the G2(MP2) and the B3LYP/6-311*G(3df,2p)//6-31G(d) approaches. In general, there is a very good agreement between calculated and experimental values for the series of N -substituted imidazoles, while the agreement is less good for the series of the N -substituted pyrazoles. Experimentally, the gap between the enthalpies of formation of imidazoles and pyrazoles decreases significantly upon N -substitution, while the theoretical estimates indicate that this decrease is smaller.