A quantum chemical mechanism for the water-initiated decomposition of silica

An ab initio quantum chemical investigation of the interaction of water with amorphous silica is carried out using model systems that contain the Si-O-Si bridging unit. Interaction of the silicon-oxygen bond with a water dimer is found to be most favorable, in contrast to the Michalske-Freiman model, which assumes that the interaction occurs with a single water molecule. Formation of a five-coordinated silicon atom is an essential intermediate in the bond fracture process, which involves proton transfer from the water dimer to the bridging oxygen, proton transfer within the water dimer, and the formation of a new Si-O bond. The activation barrier to rupture the Si-O bond in the Si-O-Si unit in the presence of water dimer is similar to30 kcal/mol. (C) 2002 Elsevier Science B.V. All rights reserved.