The structure of 3,5-bis(trifluoromethyl)pyrazole in the gas phase and in the solid state

The X-ray molecular structure of the important molecule 3,5-bis(trifluoromethyl)pyrazole has been determined at 120 K and gave crystals belonging to the triclinic PI space group. The compound forms tetramers through N-H ... N hydrogen bonds and some proton disorder is necessary to explain the geometric features of the monomers. The IR spectra have been recorded in the gas phase (monomers) and in the solid state (tetramers) and analyzed by comparison with the calculated normal frequencies. The use of solid-state NMR spectroscopy combined with ab initio CIAO calculations suggests that a certain amount (about 40 +/- 10%) of dynamic disorder involving intramolecular proton transfers could be present in the crystal.