Archive for May 19th, 2022

3-Methyl-5-phenylpyrazole: a crystallographic enigma?

Thursday, May 19th, 2022

Previously, I explored the unusual structure of a molecule with a hydrogen bonded interaction between a phenol and a pyridine. The crystal structure name was RAKQOJ and it had been reported as having almost symmetrical N…H…O hydrogen bonds. This feature had been determined using neutron diffraction crystallography, which is thought very reliable at determining proton positions. Another compound with these characteristics is 3-methyl-5-phenylpyrazole or MEPHPY01.[1] Here the neutron study showed it to apparently have the structure represented below, where the solid N-H lines indicate a proton equidistant between two nitrogens.



  1. F.H. Moore, A.H. White, and A.C. Willis, "3-Methyl-5-phenylpyrazole: a neutron diffraction study", Journal of the Chemical Society, Perkin Transactions 2, pp. 1068, 1975.