Theoretical Study of the Reaction Mechanism and Role of Water Clusters in the Gas-Phase Hydrolysis of SiCl4

The energies and thermodynamic parameters of the elementary reactions involved in the gas-phase hydrolysis of silicon tetrachloride were studied using ab initio quantum chemical methods (up to MP4//MP2/6-311G(2d,2p)), density functional (B3LYP/6-311++G(2d,2p)), and G2(MP2) theories. The proposed mechanism of hydrolysis consists of the formation of SiCl4 - x (OH) x (x = 1−4), disiloxanes Cl4 - x (OH) x -1Si−O−SiCl4 - x (OH) x -1, chainlike and cyclic siloxane polymers [−SiCl2O−] n , dichlorosilanone Cl2SiO, and silicic acid (HO)2SiO. Thermodynamic parameters were estimated, and the transition states were located for all of the elementary reactions. It was demonstrated that the experimentally observed kinetic features for the high-temperature hydrolysis are well described by a regular bimolecular reaction occurring through a four-membered cyclic transition state. In contrast, the low-temperature hydrolysis reaction cannot be described by the traditionally accepted bimolecular pathway for SiCl bond hydrolysis because of high activation barrier (E a = 107.0 kJ/mol, ΔG ⧧ = 142.5 kJ/mol) nor by reactions occurring through three- or four-molecular transition states proposed earlier for reactions occurring in aqueous solution. The transition states of SiCl4 with one- and two-coordinated water molecules were located; these significantly decrease the free energy of activation ΔG ⧧ (to 121.3 and 111.5 kJ/mol, correspondingly). However, this decrease in ΔG ⧧ is not sufficient to account for the high value of the hydrolysis rate observed experimentally under low-temperature conditions.