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

The crystal and molecular structures of 3(5),4-dimethylpyrazole, C5H8N2, (I ), and of 3,4,5-trimethylpyrazole, C6H10N2, (II), have been determined at 2 00 K. In (I) the 4,5-dimethylpyrazole tautomer is present in the solid stat e and the six independent molecules in the asymmetric unit form trimers via NH ... N hydrogen bonds related by a pseudo centre of symmetry. The asymme tric unit of (II) contains one and a half molecules: these exhibit NH proto n disorder and are hydrogen bonded to each other via their respective NH gr oups 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 m ol(-1) = 4.184 kJ mol(-1)).