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

Torres, V. and Lopez, J. M. and Langer, U. and Buntkowsky, G. and Vieth, H. M. and Elguero, J. and Limbach, H. H. (2012):
Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid.
226, In: Zeitschrift Fur Physikalische Chemie-International Journal of Research in Physical Chemistry & Chemical Physics, (11-12), pp. 1125-1147, [Online-Edition: http://apps.webofknowledge.com/full_record.do?product=WOS&se...],
[Article]

Abstract

The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of N-15 CPMAS and H-2 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 center dot center dot center dot N and an O center dot center dot center dot 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 center dot center dot ABABA center dot center dot. The longitudinal N-15 relaxation times T-1 of PCA-N-15(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 H-1-N-15 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 H-2 T-1 values of PCA-N-15(1)-d(0.9) and PCA-N-15(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 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.

Item Type: Article
Erschienen: 2012
Creators: Torres, V. and Lopez, J. M. and Langer, U. and Buntkowsky, G. and Vieth, H. M. and Elguero, J. and Limbach, H. H.
Title: Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid
Language: English
Abstract:

The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of N-15 CPMAS and H-2 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 center dot center dot center dot N and an O center dot center dot center dot 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 center dot center dot ABABA center dot center dot. The longitudinal N-15 relaxation times T-1 of PCA-N-15(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 H-1-N-15 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 H-2 T-1 values of PCA-N-15(1)-d(0.9) and PCA-N-15(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 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.

Journal or Publication Title: Zeitschrift Fur Physikalische Chemie-International Journal of Research in Physical Chemistry & Chemical Physics
Volume: 226
Number: 11-12
Uncontrolled Keywords: solid state nmr hydrogen bonded ribbon proton tautomerism order-disorder transition kinetic hydrogen/deuterium isotope effects heavy atom motions and hydrogen tunneling solid-state nmr stepwise double proton x-ray-diffraction benzoic-acid magnetic-resonance quadruple proton hh/hd/dd isotope triple proton cp/mas nmr dynamics
Divisions: 07 Department of Chemistry
07 Department of Chemistry > Physical Chemistry
Date Deposited: 27 Oct 2014 20:51
Official URL: http://apps.webofknowledge.com/full_record.do?product=WOS&se...
Additional Information:

Sp. Iss. SI 067TE Times Cited:2 Cited References Count:54

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