Theoretical study of the mechanisms of the hydrolysis and condensation reactions of silicon and titanium alkoxides: similarities and differences

Abstract

Stationary states for hydrolysis reactions in M(OCH₃)₄ + nH₂O (M = Si, Ti; n = 1–3) systems are optimized at the B3LYP and MP2 levels with the Wachters basis set for titanium and the cc-pVDZ set for other atoms. Geometries of these states for M = Ti are characterized by trigonal bipyramidal (water molecules in front-side position) and octahedral coordination (for back-side position). Barrier heights for hydrolysis and condensation are substantially lower than those for silicon in keeping with experimental results. The lowering of the barrier heights on the addition of water molecules in the front-side position (reduction of hydrogen bond strain) exceeds that of the back-side addition (catalytic effect) for both M = Si and Ti, but the difference diminishes with n. The influence of oligomerization of titanium alkoxides on the rate of hydrolysis is studied on the model of the interaction of a Ti₂(OCH₃)₈ dimer with one and two water molecules. It was shown that only terminal methoxy groups are exposed to hydrolysis and therefore the dimeric structure is retained in the process of the substitution of terminal methoxy groups. Barrier heights for terminal hydrolysis do not differ significantly from those of monomers. Barrier heights for condensation reactions obtained for the 2M(OMe)_n(OH)_4−n + H₂O model system, are substantially (by ca. 10 kcal mol⁻¹) lower for M = Ti and in both silicon and titanium species demonstrate a steady growth with n.