29Si NMR of aqueous silicate complexes at gigapascal pressures

Abstract

Geochemists have models to predict solute speciation and mineral equilibria in aqueous solutions up to 1200°C and 6 GPa. These models are useful to uncover reaction pathways deep in the Earth, though experimental confirmation is extremely difficult. Here we show speciation changes among aqueous silicate complexes to pressures of 1.8 GPa through use of a high-pressure solution-state NMR probe. The radiofrequency circuit uses a microcoil geometry that is coupled with a piston-cylinder pressure cell to generate and maintain these high pressures. The 1.8 GPa pressure corresponds to pressures reached at the lower crust or upper mantle. Although these experiments are limited to ambient temperature, we show that the increased pressure affects complexation and oligomerization reactions by eliminating bulk waters and that the pressure effects are completely reversible.

Dynamic Nuclear Polarization of Selectively 29Si-Enriched Core@shell Silica Nanoparticles

“…After 2 days of incubation in the etching solution, an average of 6 nm of particles’ shells were effectively leached and degraded into a small molecule silicate, H 3 SiO 4 – . The thickness of the etched layer was determined by SEM (Supporting Information), and the concentration of the silicate species within the etched layer was determined by intensity of the NMR peak at −72.3 ppm. , As can be seen in Figure f, both Core@Shell_20 and Silica_15 showed approximately fourfold and threefold higher intensities than the naturally abundant sample (Silica_normal), respectively, indicating a linear concentration relationship of the selective enrichments in the shell layer. However, Core_30@Shell particles exhibited the same tendency as the Silica_normal because their local spin concentrations on the etched layer were synthetically identical (i.e., natural abundance).…”

High-pressure NMR spectroscopy in studies of the conformational composition of small molecules in supercritical carbon dioxide

“…However, results given by vibrational spectroscopy are indirect, because conformer populations are derived by decomposing spectral bands into elementary components, which cannot be obtained by an unambiguous mathematical procedure; hence, vibrational spectroscopy cannot yield direct information of the fraction of conformers. An alternative method is the NMR spectroscopy at high pressures, which has shown its usefulness in studies of molecular structure [24][25][26], host-guest interactions [27], metalorganic framework [28,29], ionic liquids [30][31][32], aqueous geochemistry [33][34][35][36] and operando studies of complex mixtures [37]. In addition, liquid state NMR spectroscopy is useful in studies of conformational exchange [38][39][40][41][42].…”