29Si NMR in solid state with CPMG acquisition under MAS

Wiench, Jerzy W.; Lin, Victor S.‐Y.; Pruski, Marek

doi:10.1016/j.jmr.2008.05.007

Cited by 62 publications

(55 citation statements)

References 83 publications

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“…These effects are known to give rise to unexpected cross-peaks at moderate spinning rates and high fields in correlation experiments [37]. The recent commercial availability of very high speed MAS probes capable of spinning rates up to 70 kHz is very likely to allow new observations in the coming years, especially in protonated materials, as already demonstrated for organic solids [83], and for hybrid organic-inorganic materials with the measurements of 2 J ( 29 Si-O-29 Si) couplings in mesoporous silica ''at natural 29 Si abundance (i.e., 4.7%)'' [84].…”

Section: Towards More Accurate Measurements At High Mas Spinning Ratesmentioning

confidence: 98%

Detection and use of small J couplings in solid state NMR experiments

Massiot

Fayon

Deschamps

et al. 2009

Comptes Rendus. Chimie

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Over the years, solid-state nuclear magnetic resonance (NMR) spectroscopy has become an important tool for materials science, with its local point of view that is highly complementary to the structural information provided by diffraction techniques, electron microscopy, and molecular modeling, for example. As compared to other interactions that determine the spectral expression of the local structure of the observed nuclei in solid-state NMR experiments, the J coupling, characteristic of the chemical bonds, has received far less attention because of its being generally so small that it is masked in the line-widths. Nevertheless, the scalar or isotropic part of J couplings, which is not averaged by magic angle spinning (MAS), can be evidenced in many systems, and exploited to unequivocally characterize the extended coordination sphere. In a first step we describe the different experiments that permit the observation and the measurement of J couplings, even when dealing with quadrupolar nuclei. We then present new and recently-published results that illustrate the state of the art of NMR methodologies based on or intended for measuring J couplings in solids and the novel perspectives that they open towards better understanding of ordered and disordered materials at the subnanometric scale, a length scale that is otherwise difficult to access.

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Section: Towards More Accurate Measurements At High Mas Spinning Ratesmentioning

confidence: 98%

Detection and use of small J couplings in solid state NMR experiments

Massiot

Fayon

Deschamps

et al. 2009

Comptes Rendus. Chimie

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“…[38] Experimental parameters are given below using the following symbols: νR denotes the MAS rate, νRF(X) the magnitude of radiofrequency magnetic field (RF) applied to X spins, τCP the mixing time during cross polarization, NCPMG the number of CPMG echoes, τRD the recycle delay, NS the number of scans, and AT the total acquisition time.…”

Section: Solid-state Nmrmentioning

confidence: 99%

Substrate inhibition in the heterogeneous catalyzed aldol condensation: A mechanistic study of supported organocatalysts

Kandel

Althaus

Peeraphatdit

et al. 2012

Journal of Catalysis

137

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In this study, we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support. Disciplines Materials Chemistry | Other Chemistry | Physical ChemistryComments NOTICE: this is the author's version of a work that was accepted for publication in Journal of Catalysis. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Catalysis, [291, (2012) AbstractIn this study we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support.

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“…To accurately determine the loading of both functional groups and silanols on the MSN surface, the 29 Si NMR measurements were performed using direct polarization under MAS (DPMAS) with Carr−Purcell−Meiboom−Gill (CPMG) refocusing. 16,17 The spectra were acquired on a Chemagnetics Infinity 400 spectrometer equipped with a 5-mm MAS probe operated at 400.0 MHz ( 1 H) and 79.4 MHz ( 29 Si). The experimental parameters are given using the following notation: ν R denotes the MAS rate, ν RF (X) is the magnitude of the RF magnetic field at the frequency of X nuclei, τ CP is the mixing time during CP, N CPMG is the number of echoes acquired in CPMG experiment, τ CPMG is the corresponding time interval between π pulses, τ RD is the recycle delay, NS is the number of scans, and AT is the total acquisition time.…”

Section: ■ Introductionmentioning

confidence: 99%

Solvent-Induced Reversal of Activities between Two Closely Related Heterogeneous Catalysts in the Aldol Reaction

et al. 2013

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The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes. ABSTRACT: The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes.

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29Si NMR in solid state with CPMG acquisition under MAS

Cited by 62 publications

References 83 publications

Detection and use of small J couplings in solid state NMR experiments

Detection and use of small J couplings in solid state NMR experiments

Substrate inhibition in the heterogeneous catalyzed aldol condensation: A mechanistic study of supported organocatalysts

Solvent-Induced Reversal of Activities between Two Closely Related Heterogeneous Catalysts in the Aldol Reaction

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