A solid-state NMR, X-ray diffraction, and ab initio computational study ofhydrogen-bond structure and dynamics of pyrazole-4-carboxylic acid chains

Using high-resolution solid-state N-15 CMAS NMR, X-ray crystallography. and ab initio calculations, we have studied the structure of solid pyrazole-4- carboxylic acid (1). The crystal structure was determined at 295 and 150 K. Molecules of 1 are located on a two-fold axis. implying proton disorder of the NH and OH groups no phase transition was observed between these two te mperatures. The compound forms quasi-linear ribbons in which the molecules are linked by cyclic hydrogen bonds between pyrazole and carboxylic acid gr oups with disordered hydrogen-bonded protons. Crystallography is unable to decide whether the disorder is dynamic or static. NMR shows that this disor der is dynamic, that is, consisting of very fast degenerate double proton t ransfers between two rapidly interconverting O-(HN)-N-... and (OH)-H-...-N hydrogen bridges. However, at low temperature, NMR shows a proton disorder- order transition where the protons are preferentially localized on given ni trogen and oxygen atoms. An amorphous phase exhibiting proton order is obse rved when the compound is precipitated rapidly. In this case. the defects a re annealed by moderate heating. Ab initio calculations performed on oligom ers of 1 show that the O-(HN)-N-... hydrogen bridge is about 0.064 Angstrom shorter and less bent (similar to 171 degrees) than the (OH)-H-...-N hydro gen bridge (similar to 150 degrees). For an isolated ribbon, this result le ads to structures with localized protons. either to a cycle with about 200 molecules, or to a quasi-linear ribbon involving an undulated structure, or to a combination of both motifs. Only the undulated structure is compatibl e with the linear ribbon observed by X-ray crystallography, where the fast proton transfer in the high-temperature phase is assisted by the motions of the undulated chain. A disordered structure is assigned to the amorphous p hase, which exhibits the combination of the curved and the undulated motifs .