Enthalpies of formation of N-substituted pyrazoles and imidazoles

Accurate experimental enthalpies of formation measured using static bomb co mbustion calorimetry, the "vacuum sublimation" drop calorimetry method, and the Knudsen-effusion method are reported for the first time for four azole s: 1-methylimidazole (1MeIMI), 1-methylpyrazole (1MePYR), 1-benzylimidazole (1BnIMI), and 1-benzylpyrazole (1BnPYR). These values and those correspond ing to imidazole (1HIMI), pyrazole (1HPYR), 1-ethylimidazole (1EtIMI), 1-et hylpyrazole (1EtPYR), 1-phenylimidazole (1PhIMI), and 1-phenylpyrazole (1Ph PYR) 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 agre ement between calculated and experimental values for the series of N-substi tuted imidazoles, while the agreement is less good for the series of the N- substituted pyrazoles. Experimentally, the gap between the enthalpies of fo rmation of imidazoles and pyrazoles decreases significantly upon N-substitu tion, while the theoretical estimates indicate that this decrease is smalle r.