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Molecular orbital energy minimizations were performed with the B3LYP/6-31G(d) method on a ((OH)3SiO)3SiOH-(H3O+).4(H2O) cluster to follow the reaction path for hydrolysis of an Si-O-Si linkage via proton catalysis in a partially solvated system. The Q3 molecule was chosen (rather than Q2 or Q1) 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 (Obr). Single-point energy calculations were performed with the B3LYP/6-311+G(d,p) method. Proton transfer from the hydronium cation to an Obr requires sufficient energy to suggest that the Si-(OH)-Si species will occur only in trace quantities on a silica surface. Protonation of the Obr lengthens the Si-Obr bond and allows for the formation of a pentacoordinate Si intermediate (5Si). 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 5Si-(OH)-Si linkage with a minimal activation energy barrier. A concerted mechanism involving stretching of the 5Si-(OH) bond, proton transfer from the Si-(OH2)+ back to form H3O+, and a reversion of 5Si to tetrahedral coordination was predicted. The activation energy for Q3Si hydrolysis calculated here was found to be less than that reported for Q3Si using a constrained cluster in the literature but significantly greater than the measured activation energies for the hydrolysis of Si-Obr bonds in silicate minerals. These results suggest that the rate-limiting step in silicate dissolution is not the hydrolysis of Q3Si-Obr bonds but rather the breakage of Q2 or Q1Si-Obr bonds.
Criscenti et al. (Wed,) studied this question.