Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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Frédérique Pourpoint, Florian Venel, Raynald Giovine, Julien Trébosc, Tom Vancompernolle, Mostafa Taoufik, Vincent Sarou-Kanian, Régis M.Gauvin, Olivier Lafon, 'Probing 29Si-17O connectivities and proximities by solid-state NMR', J. Magn. Reson. 107029 (2021) doi:10.1016/j.jmr.2021.107029

The measurement of dipolar and J- couplings between 29Si and 17O isotopes is challenging owing to (i) the low abundance of both isotopes and (ii) their close Larmor frequencies, which only differ by 19%. These issues are circumvented here by the use of isotopic enrichment and dedicated triple-resonance magic-angle spinning NMR probe. The surface of 29Si-enriched silica was labelled with 17O isotope and heated at 80 and 200 °C. 29Si-17O connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments between 17O and 29Si nuclei. The simulation of the build-up of the J- and D-HMQC signals allowed the first experimental measurement of J- and dipolar coupling constants between 17O and 29Si nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q3 and Q4 groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q4 groups. The measured J- and dipolar coupling constants of siloxane 17O nucleus with Q4 29Si nuclei differ from those with Q3 29Si nuclei. These results indicate that the 29Si-17O one-bond J-coupling and Si-O bond length depend on the second neighbours of the Si atoms.