Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid
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Verónica Torres
Abstract
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.
© by Oldenbourg Wissenschaftsverlag, München, Germany
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- Editorial
- Preface
- Ampere
- NMR Study of Local and Long Range Dynamics of Adsorbed Water in Zeolite NaY(Br)
- Deuteron Magnetic Resonance Studies of Anhydrous Caffeine
- Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid
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- Melting of Low Molecular Weight Compounds in Confinement Observed by 2H-Solid State NMR: Biphenyl, a Case Study
- 14N–1H Heteronuclear Multiple-Quantum Correlation Magic-Angle Spinning NMR Spectroscopy of Organic Solids
- Multinuclear NMR Study of Zinc Dicyanide
- Generalized Routes to Mesostructured Silicates with High Metal Content
- 1H NMR T1 Relaxation of Polymer/Montmorillonite Nanocomposites with Different Clay Contents and Degrees of Exfoliation: Magnetic Field Effects
- Relaxation Exchange in Nanoporous Silica by Low-Field NMR
- Concentration and Humidity Effect on Gelatin Films Studied by NMR
- Morphology and NMR Self-Diffusion in PBA/PEO Miktoarm Star Copolymers
- Transient and Steady-State Shear Banding in a Lamellar Phase as Studied by Rheo-NMR
- Parahydrogen Induced Polarization in Scalar Coupled Systems: Analytical Solutions for Spectral Patterns and their Field Dependence
- Transfer of Parahydrogen Induced Polarization in Scalar Coupled Systems at Variable Magnetic Field
- High-Field EPR Spectroscopic Characterization of Spin Probes in Aqueous Ionic Liquid Mixtures
- Dynamical Line-Shifts in High-Field Electron Spin Resonance: Applications to Polymer Physics
- Optimization of Algorithms for Modeling Protein Structural Transitions from Sparse Long-Range Spin-Label Distance Constraints
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Articles in the same Issue
- Editorial
- Preface
- Ampere
- NMR Study of Local and Long Range Dynamics of Adsorbed Water in Zeolite NaY(Br)
- Deuteron Magnetic Resonance Studies of Anhydrous Caffeine
- Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid
- Reorientational Dynamics of Organophosphate Glass Formers – a Joint Study by 31P NMR, Dielectric Spectroscopy and Light Scattering
- Melting of Low Molecular Weight Compounds in Confinement Observed by 2H-Solid State NMR: Biphenyl, a Case Study
- 14N–1H Heteronuclear Multiple-Quantum Correlation Magic-Angle Spinning NMR Spectroscopy of Organic Solids
- Multinuclear NMR Study of Zinc Dicyanide
- Generalized Routes to Mesostructured Silicates with High Metal Content
- 1H NMR T1 Relaxation of Polymer/Montmorillonite Nanocomposites with Different Clay Contents and Degrees of Exfoliation: Magnetic Field Effects
- Relaxation Exchange in Nanoporous Silica by Low-Field NMR
- Concentration and Humidity Effect on Gelatin Films Studied by NMR
- Morphology and NMR Self-Diffusion in PBA/PEO Miktoarm Star Copolymers
- Transient and Steady-State Shear Banding in a Lamellar Phase as Studied by Rheo-NMR
- Parahydrogen Induced Polarization in Scalar Coupled Systems: Analytical Solutions for Spectral Patterns and their Field Dependence
- Transfer of Parahydrogen Induced Polarization in Scalar Coupled Systems at Variable Magnetic Field
- High-Field EPR Spectroscopic Characterization of Spin Probes in Aqueous Ionic Liquid Mixtures
- Dynamical Line-Shifts in High-Field Electron Spin Resonance: Applications to Polymer Physics
- Optimization of Algorithms for Modeling Protein Structural Transitions from Sparse Long-Range Spin-Label Distance Constraints
- First Principles Calculations of NMR Chemical Shifts of Liquid Water at an Amorphous Silica Interface
