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 ( 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 Si-(OH)-Si linkage with a minimal activation energy barrier. A concerted mechanism involving stretching of the Si-(OH) bond, proton transfer from the Si-(OH 2) + back to form H 3O +, and a reversion of 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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry