Inherent acidity of aqua metal ions in solids:: An assay in layered aluminosilicates

Structural information on 2:1 layered silicates has been obtained from H-1 magic angle spinning nuclear magnetic resonance measurements, the acidity of aqua metal ions in the interlayer space being determined. Representative end-members of the smectite group and a set of cations covering wide ranges of pK(a), absolute hardness, ionic radius, and formal charge values were carefully chosen. H-1 spectra were recorded by using conventional single-pulse MAS NMR, and their qualities were similar to those currently obtained by using multiple pulse sequences. The proton spectra of these samples were composed of two different signals, which were attributed to the structural hydroxyl groups located in the silicate lattice and the water molecules residing in the interlayer space. The position and line shape of the hydroxyl contribution was determined by the octahedral nature of the smectite and modulated by its chemical composition. The chemical shift from the water contribution gives valuable information about the acidic strength of the interlayer cation, correlations between the shielding of the protons belonging to the interlayer water molecules, and the pKa and absolute hardness values for each cation being observed.