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 SiC4-x(OH)(x) (x = 1-4), disiloxanes Cl4-x(OH)(x-1)Si-O-SiCl4-x(OH)(x-1), chainlike and cyclic siloxane polymers [-SiCl2-O-](n), dichlorosilanone Cl2Si=O, and silicic acid (HO)(2)Si=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,, = 107.0 kJ/mol, DeltaG(double dagger) = 142.5 kJ/mol) nor by reactions occurring through three- or four-molecular transition states proposed earlier for reactions occur-ring 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 AG(double dagger) (to 121.3 and 111.5 kJ/mol, correspondingly). However, this decrease in AG(double dagger) is not sufficient to account for the high value of the hydrolysis rate observed experimentally under low-temperature conditions.