Compositional effects on Si–OH bond length in hydrous silicates with implications for trends in the SiOH acidity

•Si–OH bonds in alkali hydrogen silicates are always longer than terminal Si–O bonds.•Si–OH bonds in silicates lengthen with the anionic charge per tetrahedron.•The Si–OH bond elongation results from inherent underbonding of terminal O atoms.•The longer the Si–OH bond, the less acidic the OH group is.

Theoretical calculations of the structure and Brønsted acidity of SiOH groups in silica clusters have never addressed the question if these vary with the degree of SiOH deprotonation. In this connection, a statistical analysis is presented of Si–OH bond lengths in crystalline hydrogen silicates with well-determined structures with a special emphasis placed on effects of the silicate composition. It is found that among hydrogen silicates of large cations with low charges the Si–OH bonds are always longer than terminal Si–O bonds in the same anion and correlate in length with the anionic charge per tetrahedron. The findings are explained by steric limitations on charge balancing at oxygen atoms by hydrogen bonds and/or cations. It is suggested that similar limitations and imbalances may underlie the well-known trends in the Brønsted acidity of silicic acids and silicas in aqueous media: decreased acidity with increased SiOH deprotonation and increased acidity with increased tetrahedra connectivity.

Graphical abstractSi–OH bonds in crystalline silicates lengthen with the anionic charge per tetrahedron, which is in parallel with the well-known trend of decreased acidity of silicic acids and silicas in solution with increased degree of deprotonation.Figure optionsDownload full-size imageDownload as PowerPoint slide