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Article

Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects

1
Institut für Chemie und Biochemie, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany
2
Institut für Physikalische Chemie, Universität Freiburg, Albertstr. 21, 79104 Freiburg im Breisgau, Germany
3
GKS Müllheim, Nußbaumallee 6, 79379 Müllheim, Germany
4
Klingelberg Products AS, Nordåsveien 23, 1415 Oppegård, Norway
5
Intertek AG, Tech Center Reinach, Kägenstrasse 18, CH-4153 Reinach, Switzerland
6
Department of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia
*
Author to whom correspondence should be addressed.
Academic Editor: Liudmil Antonov
Received: 24 May 2021 / Revised: 29 June 2021 / Accepted: 30 June 2021 / Published: 20 July 2021
(This article belongs to the Special Issue Tautomerism and Proton Transfer Related Phenomena)
Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d8 and in CD2Cl2. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d8 and CD2Cl2, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD2Cl2. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell–Limbach tunneling model. View Full-Text
Keywords: dynamic NMR spectroscopy; tetramethyl reductic acid; dimer-monomer equilibrium; tautomerization; double proton transfer; kinetic HH/HD isotope effects; tunneling dynamic NMR spectroscopy; tetramethyl reductic acid; dimer-monomer equilibrium; tautomerization; double proton transfer; kinetic HH/HD isotope effects; tunneling
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MDPI and ACS Style

Limbach, H.-H.; Baumgärtner, S.; Franke, R.; Männle, F.; Scherer, G.; Denisov, G.S. Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects. Molecules 2021, 26, 4373. https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26144373

AMA Style

Limbach H-H, Baumgärtner S, Franke R, Männle F, Scherer G, Denisov GS. Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects. Molecules. 2021; 26(14):4373. https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26144373

Chicago/Turabian Style

Limbach, Hans-Heinrich, Simone Baumgärtner, Roland Franke, Ferdinand Männle, Gerd Scherer, and Gleb S. Denisov 2021. "Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association, Solvent and Kinetic H/D Isotope Effects" Molecules 26, no. 14: 4373. https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26144373

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