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Hydrogen bonded silanols in the 10 Å phase: Evidence from NMR spectroscopy

¹ Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, U.S.A.
² Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60680, U.S.A.

Correspondence: ^* E-mail: brian.phillips{at}sunysb.edu

Using ²⁹Si, ¹H, and ²H magic-angle spinning (MAS) and ²⁹Si{¹H} heteronuclear correlation (Het-Cor) nuclear magnetic resonance (NMR) spectroscopy the tetrahedral sheets of the 10 Å phase are shown to contain Q²-type Si bonded to silanol groups that donate hydrogen bonds to interlayer H₂O. ²⁹Si NMR spectra of 10 Å phase samples synthesized from oxide and from crystalline talc starting materials contain a peak near –87 ppm for Q² Si, in addition to the main peak at –98 ppm for the talc-like Q³ of the tetrahedral sheet. The ¹H MAS NMR spectra of the 10 Å phase contain two distinct peaks, at chemical shifts of +7.8 and +3.2 ppm, in addition to a narrow peak near +0.9 ppm from the talc-like hydroxyl groups. ²⁹Si{¹H} HetCor data indicate that the +7.8 ppm ¹H resonance corresponds to silanol groups and that at +3.2 ppm arises from interlayer H₂O. Comparison of the observed data with correlations of ¹H NMR chemical shift and ²H quadrupolar coupling indicates that the silanol groups donate moderate hydrogen bonds to interlayer H₂O, d(O···O) 2.8 Å, whereas most interlayer H₂O donate only very weak or no hydrogen bonds at ambient conditions. Our results suggest that formation of the 10 Å phase involves formation of vacancies, which allow favorable hydrogen bond interaction between interlayer H₂O and the normally hydrophobic talc-like 2:1 layers.

Key Words: NMR • 10 Å phase • talc • silanol • hydrogen bond

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