Density functional theory investigation into structure and reactivity of prenucleation silica species

We investigated the condensation and reverse hydrolysis reactions of several silica clusters ranging in size up to the octamer cage. Using density functional theory (DFF) at the B3LYP/6-31G(d,p) level of theory, we found that under neutral conditions the reactions proceeded through a single-step, S(N)2-type mechanism with formation of a pentacoordinated transition state. Under acidic conditions, the reactions of the smaller species followed a two-step mechanism with formation of a stable pentacoordinated intermediate, while the larger species proceeded through a single-step mechanism similar to the neutral route. In vacuo energy calculations showed a decreased activation barrier for formation of the larger ringed oligomers compared to the barriers of the smaller species. Calculation also showed an increasing kinetic and thermodynamic barrier for the silica clusters to break back into smaller less-condensed constituent species as they oligomerized to form larger species. To study the influence of solvation on these reactions, we used a hybrid implicit/explicit hydration model that explicitly accounted for water in the calculations. Results on the silica dimer cluster revealed a marked change in both the mechanism and energetics of the reactions. The results suggested the presence of excess water (1) elongated the hydroxyl bonds of the cluster, (2) elongated the siloxane bonds, (3) stabilized both the reactive intermediates and transition states, and (4) acted as a proton acceptor to facilitate the reactions. The calculated rate-limiting activation barrier for the condensation reaction to form the dimer species was in excellent agreement with previous experimental and theoretical results. Application of the hybrid solvation model to calculate energy of solvation for several silica oligomers showed a decrease in solvation energy as the silica clusters grew in size.