3-Nitropyrazole, crystal structure

Generally the name of a compound should correspond to the most stable tautomer (76AHCS1, p. 5). This is often problematic when several tautomers have similar stabilities, but is a simple and reasonable rule whose violation could lead to naming phenol as cyclohexadienone. Different types of tautomerism use different types of nomenclature. For instance, in the case of annular tautomers both are named, e.g., 4(5)-methylimidazole, while for functional tautomerism, usually the name of an individual tautomer is used because to name all would be cumbersome. In the case of crystal structures, the name should reflect the tautomer actually found therefore, 3-nitropyrazole should be named as such (97JPOC637) and not as 3(5)-nitropyrazole. 

By use of 13C CP MAS NMR (crosspolarization/magic angle spinning) and X-ray diffraction the structure of 3(5)-methyl-4-nitropyrazole in the solid state was examined [252], Desmotropy in azoles has been revealed for the first time. This means that the two tautomers, 3-methyl-4-nitro- and 5-methyl-4-nitropyrazole, are crystallized in two different crystals [252, 256], The problem of proton transfer in solid-state heterocycles, including nitropyrazoles, was studied by CP MAS NMR and X-ray structural analysis [253, 257], The kinetics of degenerate intermolecular triple proton and deuteron transfers in the cyclic trimer of polycrystalline 4-nitrop-yrazole (15N-labeled) has been studied as a function of temperature and is compared to the kinetic of triple proton transfer in bulk solid 3,5-dimethylpyrazole. The results show that the transfer kinetics in the new trimer is much faster than that in 3,5-dimethylpyrazole. Nevertheless, the kinetics of HHH/HHD/HDD/DDD isotope effects of 4-nitropyrazole is similar to that of 3,5-dimethyl derivative [257]. 

The first example of desmotropy in azoles was found in the case of 3-methyl-4-nitropyrazole and its tautomer 5-methyl-4-nitropyrazole. The crystal and molecular structure of both tautomers 16a and 16b has been determined by X-ray analysis (Scheme 10). CPMAS NMR spectroscopy has been used for studying the isomerization processes in the sohd state (1994JCS(CC)1143). 

---