The structures of 1-benzyl-4-nitroso-5-aminopyrazole (1) and its hydrochloride (1H(+)) have been determined in the solid state and in solution in DMSO, methanol, and ethanol. The free base exists in solution as a mixture of amino/nitroso tautomers 2a and 2b rather than in the imino/oxime tautomers 3. The conjugated cation 1H(+) results from the protonation of the nitroso group. X-ray crystallography showed that both amino hydrogen atoms of 2a form NH...O=N hydrogen bonds: one is intramolecular, the other links adjacent molecules in an infinite chain.
Holschbach, M. H., Sanz, D., Claramunt, R. M., Infantes, L., Motherwell, S., Raithby, P. R., Jimeno, M. L., Herrero, D., Alkorta, L., Jagerovic, N., & Elguero, J. (2003). Structure of a 4-nitroso-5-aminopyrazole and its salts: Tautomerism, protonation, and E/Z isomerism. Journal of Organic Chemistry, 68(23), 8831-8837. https://doi.org/10.1021/jo034833u