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

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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 [−SiCl2O−]n, dichlorosilanone Cl2SiO, and silicic acid (HO)2SiO. 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 SiCl bond hydrolysis because of high activation barrier (Ea= 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.