A Solid-State NMR, X-ray Diffraction, and ab Initio Computational Study of Hydrogen-Bond Structure and Dynamics of Pyrazole-4-Carboxylic Acid Chains

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.

Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy

“…Some literature results concerning p-tolylimidazoles 6-9 (Scheme 2) in CDCl 3 are worth reporting [29] because we will need them to assign the 13 C signals of imidazoles 2-5, our reviews of 13 …”

“…Pyrazoles and particularly 3,5-dimethylpyrazole (1), show a dynamic process in the solid state that consists in the transfer, generally concerted, of several protons between the nitrogen atoms [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. This process (named SSPT, Solid State Proton Transfer) is illustrated in Fig.…”

The structure of 3,5-dimethylpyrazole/carboxylic acids co-crystals

“…This allowed to determine the following most important chemical shift effects: (6) is of particular relevance because pyrazole-4-carboxylic acid (8) also shows SSPT although by a different mechanism. 12 The data reported in Table 1, namely the asymmetry of the positions 3 and 5, seems to indicate that a similar phenomenon occurs in 6. The [1 + 2] mixture (1:1), actually 1 2 2 2 , was studied again.…”

Reactivity of the Pyrazolatopalladium(II) Complexes [Pd(dmpz)2(Hdmpz)2] and [Pd2(μ‐dmpz)2(dmpz)2(Hdmpz)2] towards Carboxylic Acids: Hydrogen Bonds as the Driving Force That Determines the Nature of the Final Products

“…These N-H···O hydrogen bonds seem to be quite strong as the H···O and N···O distances are perceptibly shorter than those involved in neutral molecules [23,24] and are even at the lower end of the range observed for charge-assisted N-H (+) ···O (-) hydrogen bonds. [24][25][26][27][28][29][30] According to these structural data, the C-O distances within the COO -moieties indicate that the carboxylic acid does not retain its acidic hydrogen, which has www.eurjic.org (1). [a] Pd-N(1) [a] The symmetry transformation used to generate the equivalent primed atoms is -x, -y + 1, -z + 1.…”