Water complexes and hydrolysis of silicon tetrafluoride in the gas phase: An ab initio study

An ab initio study of the structure, binding energies, electronic distribution, vibrational frequencies, and thermodynamic parameters of possible gas-phase complexes of silicon tetrafluoride with water (1:1, 2:1, and 1:2) in different conformations has been performed at the RHF, B3LYP, MP2, and MP4(sdq) levels with a variety of basis sets up to cc-pVTZ. On this basis, the thermodynamic stability, assignment of observed infrared bands of SiF4/H2O adducts in matrixes and in the gas phase, and activation energies of the elementary reactions leading to polyhydroxy derivatives and siloxane are discussed. The most thermodynamically stable structure is a 1:1 complex with the most accurate estimate of the binding energy of 2.7 kcal/mol. This complex is characterized by a small amount of electron transfer from the water molecule to silicon tetrafluoride and by small participation of the silicon d-orbitals in the coordination bond. A single stable conformation has been found for complex 2SiF(4). H2O whereas the complex SiF4. 2H(2)O can exist in two conformations (distorted octahedral symmetry), distinguished by positions of the water molecules. The energetically preferred conformation of SiF4. 2H(2)O is that with the water molecules in cis positions of an octahedron. The activation energy for the hydrolysis of first Si-F bond is 21 kcal/mol. The most thermodynamically preferred product at the first stages of hydrolysis is the hexafluorodisiloxane SiF3-O-SiF3 (activation energy 30 kcal/mol) rather than polyhydroxy derivatives.