Computational study of the adsorption of trichlorophosphate, dimethyl methylphosphonate, and sarin on amorphous SiO2

Ab initio quantum chemical calculations have been performed to study the adsorption of trichlorophosphate, dimethyl methylphosphonate, and Sarin via hydrogen bonding to Si-OH groups on the amorphous SiO2 (a-SiO2) surface. Two SiO2 models are used: a small Si5O7H8 "cagelike" cluster and a larger Si21O56H28 structure designed to approximate the local environment in a-SiO2. In the latter case, regions of different local OH density are considered as adsorption sites. Adsorption energies, bonding geometries, and adsorbate vibrational modes are obtained, and anharmonicity is explicitly included in the treatment of the SiO-H stretching mode. The computed results for the adsorption-induced shift in frequency of the SiO-H stretch and of the molecular PO stretch are compared with the available experimental data. For all three species, the most stable adsorption geometry involves hydrogen bonding between two Si-OH groups and the O atom of the PO group.