Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 441-447 [ doi:10.1107/S0108768198014013 ]

3(5),4-Dimethyl- and 3,4,5-trimethylpyrazole at 200 K. X-ray crystallography and quantum-chemical analysis

L. Infantes, C. Foces-Foces and J. Elguero

Abstract: The crystal and molecular structures of 3(5),4-dimethylpyrazole, C₅H₈N₂, (I), and of 3,4,5-trimethylpyrazole, C₆H₁₀N₂, (II), have been determined at 200 K. In (I) the 4,5-dimethylpyrazole tautomer is present in the solid state and the six independent molecules in the asymmetric unit form trimers via NHN hydrogen bonds related by a pseudo centre of symmetry. The asymmetric unit of (II) contains one and a half molecules: these exhibit NH proton disorder and are hydrogen bonded to each other via their respective NH groups to form chains. Ab initio calculations at HF and B3LYP/6-31G** levels indicate that the 3,4-dimethylpyrazole tautomer is more stable than the 4,5-dimethylpyrazole tautomer by only approximately 0.5 kcal mol^-1 (1 kcal mol^-1 = 4.184 kJ mol^-1).