Molecular dynamics of the self-organising strong hydrogen bonded 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. We have previously reported the use of quasielastic neutron scattering (QENS) to study 3,5-dimethylpyrazole which showed that the hydrogen-bonded amide protons within individual trimer units undergo a short range hopping motion between two equivalent sites straddling the direct N center dot center dot center dot H hydrogen-bond axis. This work was the first report of such a novel dynamic process in this family of materials. The current work extends the earlier study with additional QENS measurements of other isotopic variants, providing information on the methyl group dynamics, which lead us to the conclusion that the short-range amide motions are decoupled from the methyl torsions. Whilst the methyl groups were found to undergo 3-fold diffusive hopping motions on the QENS timescale (similar to 10(11) - 12(12) s(-1)), an appreciable non-mobile fraction of methyl groups was also detected at all temperatures studied.