A Solid-State NMR, X-ray Diffraction, and ab Initio Computational Study of Hydrogen-Bond Structure and Dynamics of Pyrazole-4-Carboxylic Acid Chains
- 20 July 2001
- journal article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 123 (32), 7898-7906
- https://doi.org/10.1021/ja002688l
Abstract
Using high-resolution solid-state (15)N 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 temperatures. The compound forms quasi-linear ribbons in which the molecules are linked by cyclic hydrogen bonds between pyrazole and carboxylic acid groups with disordered hydrogen-bonded protons. Crystallography is unable to decide whether the disorder is dynamic or static. NMR shows that this disorder is dynamic, that is, consisting of very fast degenerate double proton transfers between two rapidly interconverting O-H.N and O.H-N hydrogen bridges. However, at low temperature, NMR shows a proton disorder-order transition where the protons are preferentially localized on given nitrogen and oxygen atoms. An amorphous phase exhibiting proton order is observed when the compound is precipitated rapidly. In this case, the defects are annealed by moderate heating. Ab initio calculations performed on oligomers of 1 show that the O-H.N hydrogen bridge is about 0.064 A shorter and less bent ( approximately 171 degrees ) than the O.H-N hydrogen bridge ( approximately 150 degrees ). For an isolated ribbon, this result leads 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 compatible 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 phase, which exhibits the combination of the curved and the undulated motifs.Keywords
This publication has 33 references indexed in Scilit:
- The structure of 3,5-bis(trifluoromethyl)pyrazole in the gas phase and in the solid stateNew Journal of Chemistry, 1999
- Structure and Dynamics of 3,5-Di-tert-butylpyrazole Probed by Combined X-ray Crystallography and 15N Solid State NMRThe Journal of Organic Chemistry, 1995
- Intermolecular proton transfer in host–guest crystals: the case of pyrazole included in 1,1-di(2,4-dimethylphenyl)but-2-yn-1-ol, an X-ray and solid-state13C/15N NMR studyJournal of the Chemical Society, Chemical Communications, 1993
- Observation of a series of degenerate cyclic double, triple, and quadruple proton transfers in solid pyrazolesJournal of the American Chemical Society, 1992
- The dynamics of hydrogens in double well potentials: The transition of the jump rate from the low temperature quantum-mechanical to the high temperature activated regimeThe Journal of Chemical Physics, 1991
- Hydrogen bond dynamics in isotopically substituted benzoic acid dimersThe Journal of Chemical Physics, 1990
- Intermolecular triple proton and deuteron transfer in crystalline 3,5-dimethylpyrazole studied by NMR, NQR, and x-ray methodsJournal of the American Chemical Society, 1989
- Dynamic intermolecular tautomerism of 3,5-dimethylpyrazole in the solid state by carbon-13 CP/MAS NMR spectroscopy and x-ray crystallographyJournal of the American Chemical Society, 1985
- Structure and dynamics of intramolecular hydrogen bonds in carboxylic acid dimers: A solid state NMR studyThe Journal of Chemical Physics, 1982
- Annular tautomerism in the solid state: a high resolution n.m.r. studyJournal of the Chemical Society, Chemical Communications, 1981