Hydrogen Bonding Network in Interlayer Spaces of a Partially Deuterated Layered α-Sn (IV) Phosphate: A Solid-State MAS NMR Study

Abstract

Samples of a layered α-Sn (IV) phosphate were partially deuterated by soaking with D₂O to yield a mixture of two isotopomers Sn (HPO₄) (DPO₄).c-H₂O and Sn (DPO₄)₂.c-H₂O containing cavity water c-H₂O. They were characterized by the ¹H, ²H, ³¹P, and ¹¹⁹Sn MAS NMR experiments including relaxation time measurements. The formation of these isotopomers is proven by the kinetic proton–deuterium cross-polarization MAS NMR experiments giving the cross-polarization rate constant T_H-D of 3.2 ms. In agreement with their formulation the ²H MAS NMR spectra of Sn (HPO₄) (DPO₄).c-H₂O and Sn (DPO₄)₂.c-H₂O did not display the other signals besides the DPO₄ resonance. The DPO₄ groups observed in the temperature-independent ²H MAS NMR spectra show the DQCC value of 184 ± 6 kHz corresponding to hydrogen bonds formed with an O^… O distance estimated as ~2.7 Å. Because of reduced dipolar interactions in the deuterated samples, the ¹H MAS NMR spectra are well resolved providing signal assignments and the analysis. According to the solid-state NMR data collected for the partially deuterated samples of SnP, the cavity water accepts one hydrogen bond from the P-OH donor group and forms one hydrogen bond with the neighboring phosphate group, while the other water hydrogen is not involved in hydrogen bonding.