Solid-state nuclear magnetic resonance (SSNMR) has been extensively used to characterize molecular structures at atomic scales. [1] Concerning biomolecular applications, structural studies are commonly performed on 13 C and/or 15 N enriched samples to compensate for their low natural abundance (1.11 % for 13 C and 0.37 % for 15 N). However, this strategy is mainly restricted to biomolecules that can be easily isotopically enriched and has proven difficult to expand to other types of systems.To date, only a few examples of natural abundance (NA) 2D 13 C-13 C correlation experiments in solids have been reported using pulse sequences that rely on through-bond polarization transfer. [2] This type of experiment provides onebond connections and is limited to small crystalline molecules, as it requires 2 to 10 days of experimental time. Owing to the low abundance of 13 C nuclei, cross-peak intensities are about four orders of magnitude smaller for experiments performed on NA systems compared to their labeled equivalents.Recently, dynamic nuclear polarization (DNP) performed with a high-power high-frequency microwave source (gyrotron), a low-temperature (LT) magic-angle spinning (MAS) probe, and a suitable polarizing agent has emerged as an appropriate answer to the sensitivity limitation of SSNMR even at high magnetic fields. [3][4][5] This work by Griffin and coworkers has triggered a strong interest in the science community and high-field MAS-DNP has been used on many different types of systems ranging from biological systems [6][7][8] to materials. [9][10][11] Herein we will show that the sensitivity enhancement obtained with DNP can be significant enough to obtain 2D 13 C-13 C NMR correlation spectra on NA microcrystalline solids in 20 min, [12] that is, within an experimental time comparable to experiments routinely performed on isotopically labeled systems.The relevance of performing DNP experiments has so far mainly been judged by comparing the signal-to-noise ratio (S/ N) with and without microwave (MW) irradiation. This "DNP enhancement" (e DNP ) has shown factors of up to 200 at 9.4 T and 100 K, [5,13] but in most applications, a factor of 10 to 20 is obtained. However, we demonstrate here that the effective sensitivity gain in DNP experiments cannot be simply evaluated by measuring e DNP . Instead, we propose using the absolute sensitivity ratio (ASR) to evaluate the relevance of DNP by comparing the S/N per unit time obtained under optimized DNP conditions with the one obtained under standard NMR conditions (potentially using for example larger sample volumes and higher magnetic fields). Previously, Rossini et al. and Vitzthum et al. introduced an overall sensitivity factor [14] and a global DNP factor, [15] respectively, which take into account some of the important parameters. These factors are reduced forms of the ASR and are discussed in more detail in the Supporting Information, S4.One of the main differences between conventional and DNP-enhanced SSNMR experiments is the effective sample volume. For DNP...

We show how high-resolution NMR spectra can be obtained for solids for which the spectra are normally broadened due to structural disorder. The method relies on correlations in the chemical shifts between pairs of coupled spins. It is found experimentally that there are strong correlations in the chemical shifts between neighboring spins in both phosphorus-31 and carbon-13 spectra. These correlations can be exploited not only to provide resolution in two-dimensional spectra, but also to yield "chains" of correlated chemical shifts, constituting a valuable new source of structural information for disordered materials.

New asymmetrical ligands (H 2 L) have been synthesized to provide both a bridging and a terminal phenolate to a pair of iron ions in order to mimic the binding of a single terminal tyrosinate at the diiron center of the purple acid phosphatases. H 2 L1 is 2-[(bis(2-pyridylmethyl)amino)methyl]-6-[((2-pyridylmethyl)(2-phenol)amino)methyl]-4-methylphenol and H 2 L′1 and H 2 L2 are obtained by replacing the 2-phenol group by the 5-nitro-2-phenol and the 6-methyl-2-phenol residues, respectively. A series of mixed valence diiron complexes [Fe II Fe III L(X) 2 ](Y) have been obtained where (X) 2 is the dianion of m-phenylenedipropionate or (H 2 PO 4 ) 2 and Y )Diferric complexes have been obtained also either by direct synthesis or by iodine oxidation of the mixed valence precursor (L ) L1, 3a (X) 2 ) mpdp, Y ) BPh 4 , 3d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 ; L ) L2, 4d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 . Complex 1a [Fe II Fe III L(mpdp)](BPh 4 ) has been characterized by X-ray diffraction techniques. 1a crystallizes in the monoclinic space group P21/a with the following unit cell parameters: a ) 22.038 (9) Å, b ) 16.195 (8) Å, c ) 16.536 (7) Å, β ) 97.26 (1)°, Z ) 4. The significant differences in the Fe-O bond lengths indicate that the metal centers are ordered. The complexes have been studied by electronic spectral, resonance Raman, magnetic susceptibility, Mo ¨ssbauer, NMR, and electrochemical techniques. Mo ¨ssbauer and NMR spectroscopies concur to probe that the valences of the mixed valence compounds are trapped in solution as well as in the solid state at room temperature. The electronic spectrum of the mixed-valence compounds are dominated by a charge transfer transition in the 400-600 nm domain which moves to the 550-660 nm range upon oxidation to the diferric state. In addition they exhibit a weak and broad intervalence transition close to 1100 nm. Electrochemical studies show that the systems exist in the three redox states Fe

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Silica (SiO2) nanoparticles (NPs) were functionalized by silanization to produce a surface covered with organosiloxanes. Information about the surface coverage and the nature, if any, of organosiloxane polymerization, whether parallel or perpendicular to the surface, is highly desired. To this extent, two-dimensional homonuclear (29)Si solid-state NMR could be employed. However, owing to the sensitivity limitations associated with the low natural abundance (4.7%) of (29)Si and the difficulty and expense of isotopic labeling here, this technique would usually be deemed impracticable. Nevertheless, we show that recent developments in the field of dynamic nuclear polarization under magic angle spinning (MAS-DNP) could be used to dramatically increase the sensitivity of the NMR experiments, resulting in a timesaving factor of ∼625 compared to conventional solid-state NMR. This allowed the acquisition of previously infeasible data. Using both through-space and through-bond 2D (29)Si-(29)Si correlation experiments, it is shown that the required reaction conditions favor lateral polymerization and domain growth. Moreover, the natural abundance correlation experiments permitted the estimation of (2)J(Si-O-Si)-couplings (13.8 ± 1.4 Hz for surface silica) and interatomic distances (3.04 ± 0.08 Å for surface silica) since complications associated with many-spin systems and also sensitivity were avoided. The work detailed herein not only demonstrates the possibility of using MAS-DNP to greatly facilitate the acquisition of 2D (29)Si-(29)Si correlation spectra but also shows that this technique can be used in a routine fashion to characterize surface grafting networks and gain structural constraints, which can be related to a system's chemical and physical properties.

By means of a true sensitivity enhancement for a solid-state NMR spectroscopy (SSNMR) experiment performed under dynamic nuclear polarization (DNP) conditions, corresponding to 4-5 orders of magnitude of time savings compared with a conventional SSNMR experiment, it is shown that it is possible to record interface-selective (27)Al-(27)Al two-dimensional dipolar correlation spectra on mesoporous alumina, an advanced material with potential industrial applications. The low efficiency of cross-polarization and dipolar recoupling for quadrupolar nuclei is completely negated using this technique. The important presence of pentacoordinated Al has not only been observed, but its role in bridging interfacial tetra- and hexacoordinated Al has been determined. Such structural information, collected at low temperature (∼103 K) and 9.4 T with the use of DNP, would have been impossible to obtain under standard conditions, even using a higher magnetic field. However, here it is demonstrated that this information can be obtained in only 4 h. This work clearly opens a new avenue for the application of SSNMR to quadrupolar nuclei and notably the atomic-scale structure determination of catalysis materials such as mesoporous alumina.

Dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance (NMR) has recently emerged as a powerful technique for the study of material surfaces. In this study, we demonstrate its potential to investigate cell surface in intact cells. Using Bacillus subtilis bacterial cells as an example, it is shown that the polarizing agent 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL) has a strong binding affinity to cell wall polymers (peptidoglycan). This particular interaction is thoroughly investigated with a systematic study on extracted cell wall materials, disrupted cells, and entire cells, which proved that TOTAPOL is mainly accumulating in the cell wall. This property is used on one hand to selectively enhance or suppress cell wall signals by controlling radical concentrations and on the other hand to improve spectral resolution by means of a difference spectrum. Comparing DNP-enhanced and conventional solid-state NMR, an absolute sensitivity ratio of 24 was obtained on the entire cell sample. This important increase in sensitivity together with the possibility of enhancing specifically cell wall signals and improving resolution really opens new avenues for the use of DNP-enhanced solid-state NMR as an on-cell investigation tool.

The cooler the better. We report a strategy to push the limits of solid-state NMR sensitivity far beyond its current state-of-the-art.