The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of 15N CPMAS and 2H 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 15N relaxation times T1 of PCA-15N2 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 1H-15N relaxation mechanism, yielding the rate constants kHH. The rate constants kHD and kDD were obtained from the measurement and analysis of the 2H T1 values of PCA- 15N1-d0.9 and PCA-15N1-d0.1 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 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.
