Tautomerism, hydrogen bonding and vibrational properties of 4-acetyl-3(5)-amino-5(3)-methylpyrazole

The structure and molecular vibrations of 4-acetyl-3(5)-amino-5(3)-methylpy razole have been investigated by quantum chemical calculations and vibratio nal spectroscopy. The calculations predicted a predominance of the 5-amino- 3-methyl form in the tautomeric equilibrium, however. the energy difference between the two tautomers is rather small, 2 kJ/mol at the B3-LYP/6-311++G ** level of the theory. The most stable conformers of each tautomer are sta bilized by intramolecular hydrogen bonding. This interaction plays an impor tant role also in the tautomeric equilibrium contributing significantly to the stability of the 5-amino-3-methyl tautomer. Another characteristics of the hydrogen bonding is its strengthening effect on the conjugation of the NH2 and C=O groups with the pyrazole ring. The molecular vibrations of the title compound have been analysed by means of the scaled quantum mechanical method. Using joint theoretical and experimental information the FT-IR and FT-Raman spectra of solid 4-acetyl-3-amino-5-methylpyrazole have been assi gned. (C) 2001 Elsevier Science B.V. All rights reserved.