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 EHB 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 EHB by adopting the literature empirical correlation relating the Bader local electronic kinetic energy density Gb computed at the H⋅⋅⋅O bond critical point with EHB. Remarkably, EHB for the hydroxylated surfaces of quartz polymorphs correlates with surface formation energy, showing that the surface EHB is responsible of the surface stability. A number of correlations between hydrogen bond features are established, with that between EHB 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.