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

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Materials Chemistry
• Isotope Geochemistry

Background:

• Measuring dipolar and J-couplings between ²⁹Si and ¹⁷O is challenging due to low natural abundance and similar Larmor frequencies.
• Previous methods lacked the sensitivity and resolution to accurately quantify these interactions.

Purpose of the Study:

• To overcome the challenges in measuring ²⁹Si-¹⁷O couplings.
• To experimentally determine J- and dipolar coupling constants between ¹⁷O and ²⁹Si nuclei.
• To investigate the influence of neighboring silicon environments on Si-O bond characteristics.

Main Methods:

• Utilized isotopically enriched ²⁹Si silica surfaces labeled with ¹⁷O.
• Employed a triple-resonance magic-angle spinning NMR probe.
• Performed two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments.

Main Results:

• Successfully measured ²⁹Si-¹⁷O connectivities and proximities.
• First experimental determination of J- and dipolar coupling constants between ¹⁷O and ²⁹Si.
• Distinguished two distinct siloxane oxygen sites based on their silicon atom environments (Q³/Q⁴ vs. two Q⁴).
• Observed differences in coupling constants depending on the neighboring silicon atoms.

Conclusions:

• The measured ²⁹Si-¹⁷O couplings provide insights into local atomic environments in silica.
• The one-bond J-coupling and Si-O bond length are sensitive to the nature of neighboring silicon atoms.
• This methodology opens new avenues for characterizing silica surfaces and materials at the atomic level.