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(OCH(3))(4) + nH(2)O (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 Ti(2)(OCH(3))(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) + H(2)O 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.

Kinetics of Alkoxysilanes and Organoalkoxysilanes Polymerization: A Review

“…These methods, and others, have already been explained in a number of texts [15,97]. Most researchers in the field of silica used the ab initio [1,17,18,70] and DFT [3,35,71,98,99,100] methods. The ab initio method is more accurate, but takes a long computational time, while the DFT method gives acceptable accuracy with moderate computational time.…”

“…Modelling and computational chemistry are therefore used to study the kinetics of certain silane precursors to help in understanding the kinetics. This is used to build a model, which can be used to theoretically predict the suitable conditions for fabrication of a certain material [1,2,3].…”

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

“…This result also explains why H 2 O attacks oxygen rather than coordinates to the centre silicon atom with oxygen via a coordination bond. Previous DFT and ab initio calculations 8,22,26 have proved that Si(OCH 3 ) 4 also hydrolyzes through the attack of H 2 O on a Si-O bond leading to the rupture of this bond and the formation of the SiOH group and an alcohol molecule.…”

“…19 Although a variety of silica aerogels with unique properties have been fabricated, the hydrolysis and oligomerization processes can not be separated experimentally for they occur simultaneously in solution. 15,20,21 To investigate the hydrolysis and oligomerization mechanisms of silicon alkoxides, theoretical studies have been carried out by using quantum chemical calculations 8,[22][23][24][25][26] and Monte Carlo simulations. [27][28][29][30][31][32][33] The Si atoms in silicon alkoxides carry a reduced partial positive charge (e.g., d + E 0.32 in Si(OEt) 4 ), so the hydrolysis and condensation reactions of Si alkoxides need to be catalyzed, either with bases which carry strong negative charges (e.g., OH À ) or strong Lewis bases such as F À ions.…”

Titanium, zirconium, hafnium

“…Calculations on dimeric [Ti 2 (OCH 3 ) 8 ] indicate that only terminal methoxy groups are available for replacement, so hydrolysis affords a dimeric species. 42 [Ti(O i Pr) 4 ] was used as the starting material in the syntheses of [Ti(bzac) 2 (O i Pr) 2 ], [Ti{(bzac) 2 en}(O i Pr) 2 ] and [Ti(bzac1a2pol) 2 ], the last being particularly volatile for possible MOCVD application. 43 Titanium alkoxides react with carboxylic acids forming oxo-carboxylate clusters [Ti 8 O 8 (OOCR) 16 ] (R = C 6 H 5 , C(CH 3 ) 3 , CH 3 ).…”