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 Hai...ai...ai...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.