Role of F- in the hydrolysis-condensation mechanisms of silicon alkoxide Si(OCH3)4: a DFT investigation

The hydrolysis and condensation mechanisms of tetramethoxysilane aided by F- were investigated with Gaussian03 program package. The coordination of F- to tetramethoxysilane and the first-order hydrolysis of the fluorotetramethoxysilane anion were studied extensively with both CPCM full optimization and CPCM single-point energy (SPE) calculations, and the single-point energies agree well with those obtained from the full optimization with CPCM solvation model. The coordination of F- decreases the Mulliken charge on the Si atom and the energy gap between the HOMO and LUMO, and alters the shape of the frontier orbitals. Our calculations show that entropic effects elevate potential energy surface (PES) profiles distinctly, but have a minor influence on the free energy barriers. With the aid of F-, the hydrolysis barriers of fluorotetramethoxysilane anion and the dimerization free energy barriers before entropic corrections decrease from 104, 106, 109, 109 and 136, 104, 99, 94 to kJ mol(-1) to 84.2, 88.5, 77.2, 81.9 and 82.2, 80.0, 87.7, 88.7 kJ mol(-1), respectively, compared with the neutral hydrolysis and condensation barriers. These barriers are much lower than the corresponding neutral S(N)2 ones, but only slightly higher than the S(N)1 ones. The role of nucleophile F- is similar to HO-.