Thermodynamic and kinetic parameters of elementary steps in gas-phase hydrolysis of SiF₄. Quantum-chemical and FTIR spectroscopic studies

Abstract

The energies and thermodynamic parameters of elementary steps in the proposed mechanism of silicon tetrafluoride hydrolysis in the gas phase were calculated by the ab initio quantum-chemical method (MP4//MP2/6-311G(2d,2p)) and the density functional theory (B3LYP/6-311G(2d,2p)). The proposed mechanism of gas-phase hydrolysis involves the formation of mono- and dihydroxy derivatives, hexafluorodisiloxane (SiF₃OSiF₃), and linear and cyclic siloxane polymers with the chain length from three to six Si—O and difluorosilanone units. According to the calculations, all reactions considered are endothermic and are characterized by positive Gibbs free energies. The initial hydrolysis steps can be presented with a high accuracy by two parallel processes: formation of trifluorohydroxysilane (SiF₃OH) and SiF₃OSiF₃. These are the most thermodynamically favorable among all reaction channels. The transition states of these elementary steps were found and their kinetic parameters were estimated (ΔG ^≠ = 132 and 147 kJ mol^–1, respectively). The calculation results were verified using FTIR spectroscopy of a mixture of gas-phase SiF₄ and water vapor. The comparison of the theoretical (absolute) intensities of bands in the IR spectra and integral absorption coefficients in the experimental IR spectrum made it possible to calculate the equilibrium concentrations of the reactants and equilibrium constants of elementary steps of formation of SiF₃OH and SiF₃OSiF₃, which agree with the theoretical values. The role of different derivatives in deep hydrolysis and possibilities of experimental detection of particular intermediates in the gas phase were discussed.