Hydrogen-bonding in the self-organising system 3,5-dimethylpyrazole

The family of pyrazoles containing only H and CH3 substituents displays a wide variation in physical properties which can be directly related to the manner in which the molecules self-organise in the solid state. Hydrogen-bonded multimeric motifs of the substituted pyrazoles are a recurring feature of this family. The quasielastic neutron scattering data for 3,5-dimethylpyrazole presented here indicate that the hydrogen-bonded amide protons within trimer units undergo a short-range hopping motion between two equivalent sites, straddling the direct N . . .H hydrogen-bond axis. This motion is in addition to the larger tautomeric hop previously observed using NMR. The activation energy of the short-range motion has been determined herein as 1.7(1) kJ mol(-1), ca. 1/30th of that of the tautomeric proton hopping motion.