We introduce a novel NMR approach that extends the capabilities of indirect dynamic nuclear polarization (DNP) under magic-angle spinning to probe the local environment of half-integer spin ...quadrupolar nuclei. Compared to cross-polarization, this novel method based on the refocused INEPT scheme with adiabatic dipolar recoupling is easier to optimize and does not distort the quadrupolar line shapes. Furthermore, the use of this technique, instead of the PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order) scheme or direct DNP, greatly improves the sensitivity of DNP-NMR for the detection of quadrupolar isotopes with small dipolar couplings to protons, including notably those located in the subsurface of inorganic materials or with low gyromagnetic ratio (γ). This technique has been applied to identify the atomic-level structure of Brønsted acid sites of hydrated titania-supported MoO
, MoO
/TiO
, a widely used heterogeneous catalyst. The spectra of protonated and unprotonated
O sites, acquired in natural abundance, indicate the presence of various oxomolybdate species as well as HOMo
and HOMo
Brønsted acid sites. The enhanced sensitivity of this new method has also enabled the acquisition of the first DNP-enhanced spectra of
Mo and
Ti low-γ quadrupolar isotopes. This possibility has been demonstrated by detecting the signals of these nuclei near the surface of MoO
/TiO
. This technique has allowed the observation of
Ti surface sites, which are absent from the bulk region of TiO
. Furthermore, both
Mo and
Ti DNP spectra have shown an increased structural disorder of TiO
and MoO
phases near the surface of the particles and notably the preferential location of the amorphous TiO
phase at the surface of the particles. The proposed polarization transfer is also employed to acquire the first DNP-enhanced spectrum of
Zn, another low-γ quadrupolar isotope. This possibility is demonstrated for Al-doped ZnO nanoparticles used in optoelectronic devices. The obtained
O,
Al, and
Zn DNP-NMR data prove that the surface region of these nanoparticles contains ZnO phase as well as secondary phases, such as α-Al
O
and partially inverse ZnAl
O
spinel.
Recent developments in MAS DNP-NMR of materials Rankin, Andrew G.M.; Trébosc, Julien; Pourpoint, Frédérique ...
Solid state nuclear magnetic resonance,
September 2019, 2019-09-00, 20190901, Letnik:
101
Journal Article
Recenzirano
Odprti dostop
Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its ...lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B0 ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B0 field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.
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•An overview of MAS DNP-NMR in material research is presented.•MAS DNP enhances the sensitivity of solid-state NMR by several orders of magnitude.•This sensitivity gain facilitates the observation of surfaces, insensitive isotopes and diluted species.•This technique has notably been applied for pharmaceuticals, polymers, porous materials and nanoparticles.
The unambiguous characterization of different acid sites in zeolites is of great importance for understanding their catalytic performance and the rational design of highly efficient zeolite ...catalysts. In addition to various well-characterized extra-framework Al species, a tri-coordinated framework aluminum species can also serve as a Lewis acid site in zeolites, which is "NMR-invisible" owing to its extremely distorted local environment. Here we provide a feasible and reliable approach to elucidate the acidic nature of the tri-coordinated framework Al in dehydrated H-ZSM-5 zeolites
via
sensitivity-enhanced two-dimensional (2D) multiple nuclear correlation NMR experiments coupled with trimethylphosphine oxide (TMPO) probe molecules. Two types of tri-coordinated framework Al sites have been unambiguously identified, which amount to 11.6% of the total Brønsted and Lewis acid sites. Furthermore, it was found that synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the Brønsted acid site lead to the generation of superacidity (with an acid strength stronger than 100% H
2
SO
4
) in the zeolite.
A unique insight into the acidic nature of the tri-coordinated framework aluminum (Al
FR
) in H-ZSM-5 zeolite catalysts has been provided using multi-nuclear and multi-dimensional solid-state NMR spectroscopy in conjunction with TMPO probe molecules.
We recently described a family of experiments for R2n(v) Driven Spin Diffusion (RDSD) spectroscopy suitable for homonuclear correlation experiments under fast MAS conditions G. Hou, S. Yan, S.J. Sun, ...Y. Han, I.J. Byeon, J. Ahn, J. Concel, A. Samoson, A.M. Gronenborn, T. Polenova, Spin diffusion drive by R-symmetry sequencs: applications to homonuclear correlation spectroscopy in MAS NMR of biological and organic solids, J. Am. Chem. Soc. 133 (2011) 3943-3953. In these RDSD experiments, since the broadened second-order rotational resonance conditions are dominated by the radio frequency field strength and the phase shifts, as well as the size of reintroduced dipolar couplings, the different R2n(v) sequences display unique polarization transfer behaviors and different recoupling frequency bandwidths. Herein, we present a series of modified R2n(v) sequences, dubbed COmbined R2n(v)-Driven (CORD), that yield broadband homonuclear dipolar recoupling and give rise to uniform distribution of cross peak intensities across the entire correlation spectrum. We report NMR experiments and numerical simulations demonstrating that these CORD spin diffusion sequences are suitable for broadband recoupling at a wide range of magnetic fields and MAS frequencies, including fast-MAS conditions (νr=40 kHz and above). Since these CORD sequences are largely insensitive to dipolar truncation, they are well suited for the determination of long-range distance constraints, which are indispensable for the structural characterization of a broad range of systems. Using U-(13)C,(15)N-alanine and U-(13)C,(15)N-histidine, we show that under fast-MAS conditions, the CORD sequences display polarization transfer efficiencies within broadband frequency regions that are generally higher than those offered by other existing spin diffusion pulse schemes. A 89-residue U-(13)C,(15)N-dynein light chain (LC8) protein has also been used to demonstrate that the CORD sequences exhibit uniformly high cross peak intensities across the entire chemical shift range.
Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl‐containing molecules is of particular importance due to ...their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem‐diol‐type species in the activation of acetone on Sn‐β zeolite. 13C, 119Sn, and 13C–119Sn double‐resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)3Sn‐OH) on Sn‐β is responsible for the formation of the surface species. 13C MAS NMR experiments together with density functional theory calculations suggest that the gem‐diol‐type species exhibits high reactivity and can serve as an active intermediate in the Meerwein—Ponndorf–Verley–Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem‐diol‐type species offers an energy‐preferable pathway for the direct carbon‐to‐carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl‐containing molecules catalyzed by Lewis acid zeolites.
Crucial intermediate: A surface gem‐diol‐type species is identified in the activation of acetone on the open Sn site of Sn‐β zeolite by using solid‐state NMR spectroscopy. This species promotes alcohol adsorption and provides an energy‐favorable route for hydrogen transfer in the Meerwein–Ponndorf–Verley–Oppenauer reaction.
Chemically assisted swapping of labile protons by deuterons is presented for amino acids, polysaccharides, pharmaceutical compounds, and their solid formulations. Solid-state packing interactions in ...these compounds are elucidated by
1
H-
2
H isotope correlation NMR spectroscopy (iCOSY). A minuscule concentration of dopamine, 5 wt% or ∼100 μg, in a solid formulation can be detected by
2
H NMR at 28.2 T (
1
H, 1200 MHz) in under a minute.
A solid-state
1
H-
2
H isotope correlation NMR spectroscopy (iCOSY) approach is introduced to detect low concentrations of deuterated drugs in pharmaceutical formulations.
The study of the provenance of dolomitic marble artefacts has become relevant since it was discovered that quarries of this marble other than that of Cape-Vathy located on the island of Thasos have ...been exploited since Antiquity. To improve our knowledge about the provenance of materials and the extent of their dispersion, multiple archaeometric studies were performed in the past including isotope analyses, petrography, cathodoluminescence, and elemental analyses. In the present work, solid-state nuclear magnetic resonance (NMR) spectroscopy has been added to this panel of techniques. NMR allows the characterization of the material at a molecular level by looking at different nuclei: carbon, magnesium, and calcium. Statistical analysis of the data collected on both quarry samples and archaeologic items was also implemented and clearly demonstrates the efficiency of a holistic approach for provenance elucidation. Finally, the first 25Mg NMR tests have shown the potential of this technique to discriminate between dolomitic marbles of different provenance. The results are discussed in terms of their historical meaning and illustrate the exploitation of sources of dolomitic marbles other than the Greek Thasos source.
We report a new solid-state multidimensional NMR approach based on the cross-polarization with variable-contact pulse sequence Paluch, P. ; Pawlak, T. ; Amoureux, J.-P. ; Potrzebowski, M. J. J. ...Magn. Reson. 233, 2013, 56 , with 1H inverse detection and very fast magic angle spinning (νR = 60 kHz), dedicated to the measurement of local molecular motions of 1H–15N vectors. The introduced three-dimensional experiments, 1H–15N–1H and hCA(N)H, are particularly useful for the study of molecular dynamics of proteins and other complex structures. The applicability and power of this methodology have been revealed by employing as a model sample the GB-1 small protein doped with Na2CuEDTA. The results clearly prove that the dispersion of 1H–15N dipolar coupling constants well correlates with higher order structure of the protein. Our approach complements the conventional studies and offers a fast and reasonably simple method.
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