How strong are H-bonds at the fully hydroxylated silica surfaces? Insights from the B3LYP electron density topological analysis

The calculation through the supermolecular approach of the hydrogen bond strength E_HB between silanol groups at the surface of an ample class of silica-based materials is hindered by the intrinsic difficulty to define the “H-bond free” reference system. We propose, for the first time, to evaluate E_HB by adopting the literature empirical correlation relating the Bader local electronic kinetic energy density G_b computed at the H⋅⋅⋅O bond critical point with E_HB. Remarkably, E_HB for the hydroxylated surfaces of quartz polymorphs correlates with surface formation energy, showing that the surface E_HB is responsible of the surface stability. A number of correlations between hydrogen bond features are established, with that between E_HB and the enhanced infrared intensity associated to surface hydrogen bond formation, obeying the literature formula semi-quantitatively. The present results are quite general and can be extended to other inorganic surfaces where hydrogen bonds between surface sites are the dominant features.