Structure of 3(5)-methyl-4-nitropyrazole in the solid state: tautomerism, crystallography and the problem of desmotropy

The first example of desmotropy in azoles has been found concerning 3-methyl-4-nitropyrazole and its tautomer, 5-methyl-4-nitropyrazole: the crystal and molecular stucture of both tautomers has been determined by X-ray analysis, 13C CPMAS NMR spectroscopy has been used for studying the isomerization processes in the solid state.