Silicate glass and mineral dissolution: Calculated reaction paths and activation energies for hydrolysis of a Q ³ Si by H ₃O ⁺ using ab initio methods

Molecular orbital energy minimizations were performed with the B3LYP/6-31G(d) method on a [((OH) ₃SiO) ₃SiOH-(H ₃O ⁺)·4(H ₂O)] cluster to follow the reaction path for hydrolysis of an Si-O-Si linkage via proton catalysis in a partially solvated system. The Q ³ molecule was chosen (rather than Q ² or Q ¹) to estimate the maximum activation energy for a fully relaxed cluster representing the surface of an Al-depleted acid-etched alkali feldspar. Water molecules were included in the cluster to investigate the influence of explicit solvation on proton-transfer reactions and on the energy associated with hydroxylating the bridging oxygen atom (O _br). Single-point energy calculations were performed with the B3LYP/6-311+G(d,p) method. Proton transfer from the hydronium cation to an O _br requires sufficient energy to suggest that the Si-(OH)-Si species will occur only in trace quantities on a silica surface. Protonation of the O _br lengthens the Si-O _br bond and allows for the formation of a pentacoordinate Si intermediate ( ^[5]Si). The energy required to form this species is the dominant component of the activation energy barrier to hydrolysis. After formation of the pentacoordinate intermediate, hydrolysis occurs via breaking the ^[5]Si-(OH)-Si linkage with a minimal activation energy barrier. A concerted mechanism involving stretching of the ^[5]Si-(OH) bond, proton transfer from the Si-(OH ₂) ⁺ back to form H ₃O ⁺, and a reversion of ^[5]Si to tetrahedral coordination was predicted. The activation energy for Q ³Si hydrolysis calculated here was found to be less than that reported for Q ³Si using a constrained cluster in the literature but significantly greater than the measured activation energies for the hydrolysis of Si-O _br bonds in silicate minerals. These results suggest that the rate-limiting step in silicate dissolution is not the hydrolysis of Q ³Si-Obr bonds but rather the breakage of Q ² or Q ¹Si-O _br bonds.