Category Archives: DNP-SENS

DNP in Materials Science: Touching the Surface #DNPNMR

Berruyer, Pierrick, Lyndon Emsley, and Anne Lesage. “DNP in Materials Science: Touching the Surface,” 7:12, 2018.

https://doi.org/10.1002/9780470034590.emrstm1554.

Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy under magic-angle spinning has recently emerged as a unique analytical method to probe surfaces at atomic resolution. In this article, we first describe the basic principles of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). The article continues with a large review of recent literature that illustrates the versatility of this technique and its incredible potential to reveal new structural features at surfaces with details at an unprecedented level. The most recent developments, such as the application of DNP SENS to highly reactive surface sites, are finally covered.

Rossini, Aaron J. “Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy.” The Journal of Physical Chemistry Letters 9, no. 17 (September 6, 2018): 5150–59.

https://doi.org/10.1021/acs.jpclett.8b01891

High-resolution solid-state NMR spectroscopy is a powerful tool for the study of organic and inorganic materials because it can directly probe the symmetry and structure at nuclear sites, the connectivity/bonding of atoms and precisely measure inter-atomic distances. However, NMR spectroscopy is hampered by intrinsically poor sensitivity, consequently, the application of NMR spectroscopy to many solid materials is often infeasible. High-field dynamic nuclear polarization (DNP) has emerged as a technique to routinely enhance the sensitivity of solid-state NMR experiments by one to three orders of magnitude. This perspective gives a general overview of how DNP-enhanced solid-state NMR spectroscopy can be applied to a variety of inorganic and organic materials. DNP-enhanced solid-state NMR experiments provide unique insights into the molecular structure, which makes it possible to form structure-activity relationships that ultimately assist in the rational design and improvement of materials.

Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy (DNP SENS): Principles, protocols, and practice #DNPNMR

Liao, Wei-Chih, Behnaz Ghaffari, Christopher P. Gordon, Jun Xu, and Christophe Copéret. “Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS): Principles, Protocols, and Practice.” Current Opinion in Colloid & Interface Science 33 (January 1, 2018): 63–71.

https://doi.org/10.1016/j.cocis.2018.02.006.

Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy has been demonstrated to significantly improve NMR sensitivity on materials by 1 to 2 orders of magnitude at high magnetic fields. The preferential surface enhancement also allows for selectively probing the solid surface. In this review, we will briefly describe the main mechanism used nowadays, i.e. cross effect, for DNP enhanced solid-state NMR. We will show the typical protocols of sample formulation leading to effective DNP surface enhancements and the key experimental factors in performing DNP SENS experiments. Other important developments in DNP, i.e. shielded polarizing agents for reactive surfaces, hyperpolarizing solid matrices, and high-temperature and high-field DNP, will be discussed as well. Finally, we close the review with a short summary and our perspectives on the directions of future developments in this field.

Optimal sample formulations for DNP SENS: The importance of radical-surface interactions #DNPNMR

Perras, F.A., et al., Optimal sample formulations for DNP SENS: The importance of radical-surface interactions. Current Opinion in Colloid & Interface Science, 2018. 33: p. 9-18.

http://www.sciencedirect.com/science/article/pii/S1359029417300894

The efficacy of dynamic nuclear polarization (DNP) surface-enhanced NMR spectroscopy (SENS) is reviewed for alumina, silica, and ordered mesoporous carbon (OMC) materials, with vastly different surface areas, as a function of the biradical concentration. Importantly, our studies show that the use of a “one-size-fits-all” biradical concentration should be avoided when performing DNP SENS experiments and instead an optimal concentration should be selected as appropriate for the type of material studied as well as its surface area. In general, materials with greater surface areas require higher radical concentrations for best possible DNP performance. This result is explained with the use of a thermodynamic model wherein radical-surface interactions are expected to lead to an increase in the local concentration of the polarizing agent at the surface. We also show, using plane-wave density functional theory calculations, that weak radical-surface interactions are the cause of the poor performance of DNP SENS for carbonaceous materials.

Three-Dimensional Structure Determination of Surface Sites #DNPNMR #NMR

Berruyer, P., et al., Three-Dimensional Structure Determination of Surface Sites. J Am Chem Soc, 2017. 139(2): p. 849-855.

https://www.ncbi.nlm.nih.gov/pubmed/27997167

The spatial arrangement of atoms is directly linked to chemical function. A fundamental challenge in surface chemistry and catalysis relates to the determination of three-dimensional structures with atomic-level precision. Here we determine the three-dimensional structure of an organometallic complex on an amorphous silica surface using solid-state NMR measurements, enabled through a dynamic nuclear polarization surface enhanced NMR spectroscopy approach that induces a 200-fold increase in the NMR sensitivity for the surface species. The result, in combination with EXAFS, is a detailed structure for the surface complex determined with a precision of 0.7 A. We observe a single well-defined conformation that is folded toward the surface in such a way as to include an interaction between the platinum metal center and the surface oxygen atoms.

Natural Abundance 17 O DNP NMR Provides Precise O-H Distances and Insights into the Bronsted Acidity of Heterogeneous Catalysts #DNPNMR

Perras, F.A., et al., Natural Abundance 17 O DNP NMR Provides Precise O-H Distances and Insights into the Bronsted Acidity of Heterogeneous Catalysts. Angew Chem Int Ed Engl, 2017. 56(31): p. 9165-9169.

https://www.ncbi.nlm.nih.gov/pubmed/28499071

Heterogeneous Bronsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Bronsted acid sites can be directly observed at natural abundance by 17 O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O-H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O-H bonds.

Probing Surface Hydrogen Bonding and Dynamics by Natural Abundance, Multidimensional,17O DNP-NMR Spectroscopy #DNPNMR

Perras, F.A., et al., Probing Surface Hydrogen Bonding and Dynamics by Natural Abundance, Multidimensional,17O DNP-NMR Spectroscopy. The Journal of Physical Chemistry C, 2016. 120(21): p. 11535-11544.

http://dx.doi.org/10.1021/acs.jpcc.6b02579

Dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy is increasingly being used as a tool for the atomic-level characterization of surface sites. DNP surface-enhanced SSNMR spectroscopy of materials has, however, been limited to studying relatively receptive nuclei, and the particularly rare 17O nuclide, which is of great interest for materials science, has not been utilized. We demonstrate that advanced 17O SSNMR experiments can be performed on surface species at natural isotopic abundance using DNP. We use 17O DNP surface-enhanced 2D SSNMR to measure 17O{1H} HETCOR spectra as well as dipolar oscillations on a series of thermally treated mesoporous silica nanoparticle samples having different pore diameters. These experiments allow for a nonintrusive and unambiguous characterization of hydrogen bonding and dynamics at the surface of the material; no other single experiment can give such details about the interactions at the surface. Our data show that, upon drying, strongly hydrogen-bonded surface silanols, whose motions are greatly restricted by the interaction when compared to lone silanols, are selectively dehydroxylated.

Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy #DNPNMR

Perras, F.A., et al., Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy. J Am Chem Soc, 2017. 139(7): p. 2702-2709.

https://www.ncbi.nlm.nih.gov/pubmed/28112506

The characterization of nanometer-scale interactions between carbon-containing substrates and alumina surfaces is of paramount importance to industrial and academic catalysis applications, but it is also very challenging. Here, we demonstrate that dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) allows the unambiguous description of the coordination geometries and conformations of the substrates at the alumina surface through high-resolution measurements of 13C-27Al distances. We apply this new technique to elucidate the molecular-level geometry of 13C-enriched methionine and natural abundance poly(vinyl alcohol) adsorbed on gamma-Al2O3-supported Pd catalysts, and we support these results with element-specific X-ray absorption near-edge measurements. This work clearly demonstrates a surprising bimodal coordination of methionine at the Pd-Al2O3 interface.

Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS #DNPNMR

Ong, T.C., et al., Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS. Angew Chem Int Ed Engl, 2016. 55(15): p. 4743-7.

https://www.ncbi.nlm.nih.gov/pubmed/26953812

Obtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction-based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy (SENS) towards surface species in heterogeneous alkene metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles, which are the cycloaddition intermediates in the alkene metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous metathesis catalysts.

Insights into the catalytic activity of nitridated fibrous silica (KCC-1) nanocatalysts from (15) N and (29) Si NMR spectroscopy enhanced by dynamic nuclear polarization

Lilly Thankamony, A.S., et al., Insights into the catalytic activity of nitridated fibrous silica (KCC-1) nanocatalysts from (15) N and (29) Si NMR spectroscopy enhanced by dynamic nuclear polarization. Angew Chem Int Ed Engl, 2015. 54(7): p. 2190-3.

http://www.ncbi.nlm.nih.gov/pubmed/25469825

Fibrous nanosilica (KCC-1) oxynitrides are promising solid-base catalysts. Paradoxically, when their nitrogen content increases, their catalytic activity decreases. This counterintuitive observation is explained here for the first time using (15) N-solid-state NMR spectroscopy enhanced by dynamic nuclear polarization.

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