Solid-state NMR study of locations and dynamics of interlayer cationsand water in kanemite

Abstract

The locations and dynamics of interlayer sodium (Na) cations and water in kanemite have been studied by means of solid-state ¹ H, ²⁹ Si and ²³ Na NMR. In addition to one-dimensional (1D) spectra, ¹ H– ²⁹ Si two-dimensional (2D) cross-polarization (CP) correlation, ²³ Na 2D nutation and ²³ Na 2D triple-quantum spectra were also measured. Hydroxy groups have a large ¹ H chemical shift (15 ppm from tetramethylsilane), forming strong hydrogen bonds of the Si–O–HO ^- –Si type between the layers. When the amount of water is large, the water molecules are mobile in the interlayer space. With a decrease in the water content, the motion of the water is restricted. There is only one type of Si site in undried kanemite. The decrease in the water content affects the layer structure, which is indicated by a new ²⁹ Si peak. The site of Na ⁺ species has an isotropic chemical shift of 2.9 ppm from 1 mol dm ^-3 NaCl aqueous solution, a quadrupole coupling constant of 2.05 MHz and an asymmetry factor of 0.63. The Na ⁺ species do not rotate isotropically, but have a fixed orientation. Water molecules do not coordinate to the Na ions directly.