4 research outputs found

    Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy

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    Dynamic nuclear polarization (DNP) is shown to greatly improve the solid-state nuclear magnetic resonance (SSNMR) analysis of synthetic polymers by allowing structural assignment of intrinsically diluted NMR signals, which are typically not detected in conventional SSNMR. Specifically, SSNMR and DNP SSNMR were comparatively used to study functional polymers for which precise structural elucidation of chain ends is essential to control their reactivity and to eventually obtain advanced polymeric materials of complex architecture. Results show that the polymer chain-end signals, while hardly observable in conventional SSNMR, could be clearly identified in the DNP SSNMR spectrum owing to the increase in sensitivity afforded by the DNP setup (a factor āˆ¼10 was achieved here), hence providing access to detailed structural characterization within realistic experimental times. This sizable gain in sensitivity opens new avenues for the characterization of ā€œsmartā€ functional polymeric materials and new analytical perspectives in polymer science

    Optimizing Sample Preparation Methods for Dynamic Nuclear Polarization Solid-state NMR of Synthetic Polymers

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    This work compares the overall sensitivity enhancements provided by dynamic nuclear polarization (DNP) for the solid-state NMR characterization of polymer samples doped with biradicals and prepared either by <i>film casting</i> (FC), or by <i>glass forming</i> (GF) using 1,1,2,2-tetrachloroethane as the solvent. Analysis of amorphous and semicrystalline polymers (polystyrene, polyĀ­(ethylene oxide), polylactide, polyĀ­(methyl methacrylate)) of varying molecular weights showed that GF provided larger sensitivity enhancements than FC but yielded DNP-enhanced <sup>13</sup>C CPMAS spectra of lower resolution for semicrystalline polymers, owing to line-broadening due to conformational distribution of the polymer chains in frozen solution. Moreover, use of deuterated solvents significantly reduced the intensity of the solvent signals in the DNP-enhanced <sup>13</sup>C CPMAS spectra of polymers prepared by GF, while preserving the sensitivity enhancement observed for the polymer signals. For the polymers investigated here, both FC and GF performed better than <i>incipient wetness impregnation</i>, yielding overall sensitivity enhancements between 5 and 40

    Spin Exchange Monitoring of the Strong Positive Homotropic Allosteric Binding of a Tetraradical by a Synthetic Receptor in Water

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    The flexible tetranitroxide <b>4T</b> has been prepared and was shown to exhibit a nine line EPR spectrum in water, characteristic of significant through space spin exchange (<i>J</i><sub><i>ij</i></sub>) between four electron spins interacting with four nitrogen nuclei (<i>J</i><sub><i>ij</i></sub> ā‰« <i>a</i><sub>N</sub>). Addition of CB[8] to <b>4T</b> decreases dramatically all the <i>J</i><sub><i>ij</i></sub> couplings, and the nine line spectrum is replaced by the characteristic three line spectrum of a mononitroxide. The supramolecular association between <b>4T</b> and CB[8] involves a highly cooperative asymmetric complexation by two CB[8] (<i>K</i><sub>1</sub> = 4027 M<sup>ā€“1</sup>; <i>K</i><sub>2</sub> = 202ā€Æ800 M<sup>ā€“1</sup>; Ī± = 201) leading to a rigid complex with remote nitroxide moieties. The remarkable enhancement for the affinity of the second CB[8] corresponds to an allosteric interaction energy of ā‰ˆ13 kJ mol<sup>ā€“1</sup>, which is comparable to that of the binding of oxygen by hemoglobin. These results are confirmed by competition and reduction experiments, DFT and molecular dynamics calculations, mass spectrometry, and liquid state NMR of the corresponding reduced complex bearing hydroxylamine moieties. This study shows that suitably designed molecules can generate allosteric complexation with CB[8]. The molecule must (i) carry several recognizable groups for CB[8] and (ii) be folded so that the first binding event <i>reorganizes</i> the molecule (unfold) for a better subsequent recognition. The presence of accessible protonable amines and H-bond donors to fit with the second point are also further stabilizing groups of CB[8] complexation. In these conditions, the spin exchange coupling between four radicals has been efficiently and finely tuned and the resulting allosteric complexation induced a dramatic stabilization enhancement of the included paramagnetic moieties in highly reducing conditions through the formation of the supramolecular <b>4T</b>@CBĀ­[8]<sub>2</sub> complex

    Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K

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    A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (Īµ) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger Īµ. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in Īµ between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on Īµ is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 gĀ·mol<sup>ā€“1</sup>, is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large Īµ may be obtained at higher temperatures
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