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Multiple oxygen sites in synthetic phyllosilicates with expandable layers: ¹⁷O solid-state NMR study

¹ School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea
² U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180, U.S.A.

Correspondence: ^* E-mail: sungklee{at}snu.ac.kr

Recent advances in high-resolution solid-state NMR enabled us to resolve the previously unknown details of atomic configurations around quadrupolar nuclides such as O (¹⁷O, spin 5/2) and aluminum (²⁷Al, spin 5/2) in phyllosilicates. Here, we report the ¹⁷O 1D and 2D NMR (MAS and MQMAS) spectra of synthetic kaolinite-montmorillonite mixtures. Various O sites in the mixtures revealed from O-17 NMR include apical O (^VISi-O-2^IVAl), multiple basal O atoms (^VISi-O-^VISi and two ^VISi-O^–VIAl sites) in tetrahedral layers, and hydroxyl group with distinct ranges for NMR parameters (quadrupolar coupling constant, C_q and isotropic chemical shift, ¹⁷O _iso). In particular, the structurally relevant NMR parameters (¹⁷O _iso, C_q, and asymmetry parameter, ) for two resolved ^IVSi-O-^VIAl sites are about 52 ± 2 ppm, 2.6 ± 0.2 MHz, and 0.5 for ^IVSi-O-^IVAl (I) and about 44.5 ± 2 ppm, and 3.1 ± 0.2 MHz, and 0.5 for ^IVSi-O-^IVAl (II), respectively. These sites may stem from crystallographically distinct ^IVSi-O-^IVAl sites of varying bond length and ^IVSi-O-^VIAl bond angle or ^IVAl-O-2^VIAl site in the montmorillonite.

Key Words: High-resolution solid-state NMR • phyllosilicates • oxygen sites • 3QMAS NMR • montmorillonite

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