Theoretical Study of the Reaction Mechanism and Role of Water Clusters in the Gas-Phase Hydrolysis of SiCl_4

摘要

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 hydrolysius consists of the formation of SiCl_(4-x)(OH)_x (x = 1 - 4), disiloxanes Cl_(4-x)(OH)_(x-1)Si-O-SiCl_(4-x)(OH)_(x-1), chainlike and cyclic siloxane polymers [-SiCl_2-O-]_n, dichlorosilanone Cl_2Si=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 SiCl_4 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, respectively). However, this decrease in ΔG~(++) is not sufficient to account for the high value of the hydrolysis rate observed experimentally under low-temperature conditions.