Probing 29Si-17O connectivities and proximities by solid-state NMR

The measurement of dipolar and J- couplings between Si-29 and O-17 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 magicangle spinning NMR probe. The surface of Si-29-enriched silica was labelled with O-17 isotope and heated at 80 and 200 degrees C. Si-29-O-17 connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments between O-17 and Si-29 nuclei. The simulation of the build-up of the J- and D-HMQC signals allowed the first experimental measurement of k and dipolar coupling constants between O-17 and Si-29 nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q(3) and Q(4) groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q(4) groups. The measured J- and dipolar coupling constants of siloxane O-17 nucleus with Q4 Si-29 nuclei differ from those with Q(3) Si-29 nuclei. These results indicate that the Si-29-O-17 one-bond kcoupling and Si-O bond length depend on the second neighbours of the Si atoms. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

The measurement of dipolar and J-couplings between Si-29 and O-17 isotopes was successfully carried out in this study using isotopic enrichment and a dedicated NMR probe. The results showed different coupling constants and bond lengths between siloxane O-17 nucleus and Q4 Si-29 nuclei compared to Q3 Si-29 nuclei, indicating the dependence of Si-29-O-17 coupling and Si-O bond length on the second neighbors of the Si atoms.