Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid

Abstract

The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of 15 N CPMAS and 2 H NMR spectroscopy and relaxometry. Down to 250 K, PCA forms a hydrogen bonded ribbon where adjacent carboxylic and pyrazole groups are linked by an OH· · ·N and an O· · ·HN hydrogen bond, forming either the tautomeric state A or B. Down to about 250 K, the tautomerism is fast on the NMR timescale and degenerate, corresponding to a phase exhibiting dynamic proton disorder. At lower temperatures, a transition to an ordered phase is observed with localized protons, assigned to an all-syn conformation adopting the sequence of tautomeric states ..ABABA.. The longitudinal 15 N relaxation times T 1 of PCA-15 N 2 have been measured at 9.12 MHz (2.1 T). Because of the low field, a chemical shift anisotropy mechanism could be neglected, and the data were analyzed in terms of a dipolar 1 H-15 N relaxation mechanism, yielding the rate constants k HH . The rate constants k HD and k DD were obtained from the measurement and analysis of the 2 H T 1 values of PCA-15 N 1 -d 0.9 and PCA-15 N 1 -d 0.1 measured at 46.03 MHz. Within the margin of error, no kinetic isotope effects could be detected, in contrast to previous results reported for the very fast tautomerism of solid benzoic acid dimers and the much slower tautomerism of solid 3,5-diphenyl-4-brompyrazole (DPBrP) dimers. The Arrhenius curves of all three systems were simulated using the Bell-Limbach tunneling model. Evidence for a major heavy atom motion for the tautomerism of PCA is obtained, associated with small angle reorientation of PCA molecules Brought to you by | University of California Authenticated Download Date | 8/25/15 5:13 PM 1126 V. Torres et al.around the molecular axis. The observed proton order-disorder transition and the mechanism of the observed rate process are discussed in terms of a coupling of adjacent tautomeric states.

Kinetics of low-temperature transitions and a reaction rate theory from non-equilibrium distributions

“…Within an entirely different chemical environment, experimental rate constants related to multiple proton transfer reactions in condensed matter also show strong sub-Arrhenius behaviour [102][103][104]. Also, sub-Arrhenius behaviour has been revealed for rates of processes promoted by enzymatic catalysis [55,68,105].…”

“…Concerning atmospheric and industrial reactions, the direct H-atom abstraction from organic compounds by the hydroxyl radical often presents an evident concave curvature in the Arrhenius plot [98][99][100][101]. Within an entirely different chemical environment, experimental rate constants related to multiple proton transfer reactions in condensed matter also show strong sub-Arrhenius behaviour [102][103][104]. Also, sub-Arrhenius behaviour has been revealed for rates of processes promoted by enzymatic catalysis [55,68,105].…”

Cyclic trimers of phosphinic acids in polar aprotic solvent: symmetry, chirality and H/D isotope effects on NMR chemical shifts

“…Such self-associates can be formed between molecules in the gas-phase, in the liquid-state and in molecular crystals. 1,2 Low-molecular weight organic compounds such as pyrazoles, 3,4 amidines, [5][6][7] carboxylic acids [8][9][10][11] and others are known to form various hydrogen-bonded self-associates. Among macromolecules, perhaps the most famous examples of self-association are the DNA double helix 12 and protein dimerisation.…”

Self-Assembly of Hydrogen-Bonded Cage Tetramers of Phosphonic Acid

“…Self-association via multiple hydrogen bonds is characteristic for low-molecular weight compounds that exhibit both proton-donating and proton-accepting functional groups in mutual orientations that do not prevent several hydrogen bonds forming at the same time. Examples of such compounds include pyrazoles [1,2], amidines [3][4][5], and carboxylic acids [6][7][8][9]. Linear or cyclic self-associates, such as dimers, trimers, and tetramers, can be formed between the molecules in the gas or condensed phases [10,11].…”