Strained ring motif at silica surfaces: A quantum mechanical study of their reactivity towards protic molecules

The ring opening reactions of (SiO)(n) ring motifs (n = 2, 3 and 4) at the silica surfaces by the interaction with H2O (forming two SiOH silanol groups) and HCOOH (forming a SiOH group and a Si-O-C(=O)-H surface mixed anhydride group) has been studied by means of quantum chemical methods. Results are based on a cluster approach adopting the B3LYP-D2/6-311++G(d,p) level of calculations for computing the energetics of reaction. All reactions envisage an activated complex in which the nucleophilic part of the molecules (O atom for H2O and the carbonyl O atom for HCOOH) attacks the Si atoms of the (SiO) moiety followed by a proton transfer from the molecules towards an O atom of the ring. Opening of the most strained two-membered (SiO)(2) ring by the considered molecules is highly exoergic. For the medium strained three-membered (SiO)(3) ring, two different Si atoms can react, one exposed to the exterior part of the surface and the other buried in the inner region of the surface. Reaction with the outer Si atom is exoergic, whereas that with the inner Si atom is endoergic, indicating the role of possible constraints enforced on the reaction products by the silica surroundings. Opening the regular four-membered (SiO)(4) ring is an endoergic process for the both probe molecules. Calculations also show that the energy barriers for reactions with HCOOH are lower than with H2O because the transition state structures involving HCOOH exhibited a 6-membered ring, far less strained than the 4-membered ring found with H2O. A further reason is the more acidic character of the HCOOH compared to H2O, favoring the proton release towards the silica surface oxygen. Furthermore, it was shown that the energy barriers of the ring opening reactions with H2O is significantly lowered by the presence of an additional H2O molecule acting through the proton relay mechanism. Reaction energies, in contrast, are less favorable with HCOOH than with H2O, thus indicating that the formation of SiOH groups is favored with respect to the Si-O-C(=O)-H moiety. Finally, population analysis indicate that the electrophilic character of the C atom in the Si-O-C(=O)-H moiety is higher than for the isolated HCOOH, thereby making it more prone to undergo nucleophilic attack bringing an easier formation of amides.