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NMR studies of double proton transfer in hydrogen bonded cyclic N,N '-diarylformamidine dimers: conformational control, kinetic HH/HD/DD isotope effects and tunneling

Lopez, J. M. and Männle, F. and Wawer, I. and Buntkowsky, G. and Limbach, H. H. (2007):
NMR studies of double proton transfer in hydrogen bonded cyclic N,N '-diarylformamidine dimers: conformational control, kinetic HH/HD/DD isotope effects and tunneling.
9, In: Physical Chemistry Chemical Physics, (32), pp. 4498-4513, [Online-Edition: http://apps.webofknowledge.com/full_record.do?product=WOS&se...],
[Article]

Abstract

Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline N-15,N-15 '-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of N-15 NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal N-15 and H-2 relaxation measurements. N-15 CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline N-15,N-15 '-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and N-15,N-15 '-di-(4fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell-Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.

Item Type: Article
Erschienen: 2007
Creators: Lopez, J. M. and Männle, F. and Wawer, I. and Buntkowsky, G. and Limbach, H. H.
Title: NMR studies of double proton transfer in hydrogen bonded cyclic N,N '-diarylformamidine dimers: conformational control, kinetic HH/HD/DD isotope effects and tunneling
Language: English
Abstract:

Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline N-15,N-15 '-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of N-15 NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal N-15 and H-2 relaxation measurements. N-15 CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline N-15,N-15 '-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and N-15,N-15 '-di-(4fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell-Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.

Journal or Publication Title: Physical Chemistry Chemical Physics
Volume: 9
Number: 32
Uncontrolled Keywords: nuclear magnetic-resonance n-15 cpmas nmr free-base porphyrin solid-state benzoic-acid dynamics calculation chemical-exchange quadruple proton condensed-phase acetic-acid
Divisions: 07 Department of Chemistry
07 Department of Chemistry > Physical Chemistry
Date Deposited: 27 Oct 2014 20:45
Official URL: http://apps.webofknowledge.com/full_record.do?product=WOS&se...
Additional Information:

207ZM Times Cited:21 Cited References Count:60

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