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Isotope and Phase Effects on the Proton Tautomerism in Polycrystalline Porphycene Revealed by NMR

del Amo, J. M. L. and Langer, U. and Torres, V. and Pietrzak, M. and Buntkowsky, G. and Vieth, H. M. and Shibl, M. F. and Kuhn, O. and Broring, M. and Limbach, H. H. (2009):
Isotope and Phase Effects on the Proton Tautomerism in Polycrystalline Porphycene Revealed by NMR.
In: Journal of Physical Chemistry A, pp. 2193-2206, 113, (10), [Online-Edition: http://apps.webofknowledge.com/full_record.do?product=WOS&se...],
[Article]

Abstract

Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates.

Item Type: Article
Erschienen: 2009
Creators: del Amo, J. M. L. and Langer, U. and Torres, V. and Pietrzak, M. and Buntkowsky, G. and Vieth, H. M. and Shibl, M. F. and Kuhn, O. and Broring, M. and Limbach, H. H.
Title: Isotope and Phase Effects on the Proton Tautomerism in Polycrystalline Porphycene Revealed by NMR
Language: English
Abstract:

Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates.

Journal or Publication Title: Journal of Physical Chemistry A
Volume: 113
Number: 10
Uncontrolled Keywords: kinetic hh/hd/dd isotope solid-state nmr hydrogen-bond dynamics nuclear-magnetic-resonance stepwise double proton n-15 cpmas nmr ground-state deuteron transfer supersonic jet benzoic-acid
Divisions: 07 Department of Chemistry
07 Department of Chemistry > Physical Chemistry
Date Deposited: 27 Oct 2014 20:38
Official URL: http://apps.webofknowledge.com/full_record.do?product=WOS&se...
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415ZK Times Cited:20 Cited References Count:75

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