THE ACIDITY OF SURFACE SILANOL GROUPS - A THEORETICAL ESTIMATE BASED ON AB-INITIO CALCULATIONS ON A MODEL SURFACE

Quantum chemical SCF calculations on a double four-membered silicate ring model, H8Si8O20, of silica yield a theoretical estimate of 1400 +/- 25 kJ/mol for the heat of deprotonation of surface silanol groups, in close agreement with a value of 1390 +/- 25 kJ/mol inferred from spectroscopy. The predicted H-1 NMR chemical shift of 2.3 ppm falls in the range of shifts (1.8-2.3 ppm) observed for surface silanol groups. When comparison is made with the results for the silanol molecule (the smallest model of a surface silanol group) differences as large as 106 kJ/mol and 1.4 ppm emerge for the heat of deprotonation and the H-1 NMR chemical shift, while the OH stretching force constant is virtually identical. This shows that the convergence of predictions for surface sites with the size of the model used in the calculations strongly depends on the property considered.