Theoretical investigations into the nucleation of silica growth in basic solution part I ― ab Initio studies of the formation of trimers and tetramers

We have performed the first ab initio study that considers all possible oligomerization reactions available to silicic acid in basic solution (ignoring high-energy multiply charged species), up to and including tetramers. Energies have been computed at the MP2/6-31+G(d)//HF/6-31+G(d) level of theory, with a hybrid solvation model which includes explicit waters and CPCM continuum corrections to account for solvent effects. The explicit waters have been found to be extremely important in providing additional stabilization of intermediates and transition states which are not accounted for properly by more conventional implicit solvent approaches, suggesting some of the known pathways are more facile than previously predicted. Additionally, a number of previously neglected bimolecular growth pathways such as dimer/dimer and monomer/cyclic trimer additions, and unimolecular cyclization steps such as branched 3-ring formation from branching tetramers and the formation of highly condensed bicyclic and tetrahedral clusters have been studied in detail. Many are found to be very energetically feasible and consequently could have considerable impacts on the initial stages of silica nucleation.