Adsorption of sarin and soman on dickite: An ab initio ONIOM study

The adsorption of sarin (GB), isopropyl methylphosphonofluoridate (C4H10FO2P), and soman (GD), 3,3-dimethyl-2-butyl methylphosphonoflouridate (C7H16FO2P), on the octahedral and tetrahedral surfaces of dickite was investigated using the ONIOM(B3LYP/6-31G(d,p):PM3) and ONIOM(B3LYP/6-31G(d,p):HF/3-21G) methods and the representative cluster models. In the case of adsorption on the octahedral sheet of dickite, the location of six hydrogen atoms of the outer -OH groups and the geometry of the adsorbed molecules were optimized. In the case of adsorption on the tetrahedral side of the dickite layer, the geometry of the mineral fragment was kept frozen. The location and orientation of GB and GD on the tetrahedral and octahedral surfaces of dickite were found. The adsorption on the surface of minerals occurs because of the formation of multiple hydrogen bonds between adsorbed GB and GD and the hydroxyl groups (the octahedral side) and the basal oxygen atoms (the tetrahedral side). This type of adsorption results in the polarization and the electron density redistribution of GB and GD on the surface of the mineral. The interaction energies of sarin and soman with the octahedral and tetrahedral surfaces corrected by the BSSE energy were found. The adsorption energies obtained at the ONIOM(B3LYP/6-31G(d,p):PM3) level of theory and using large models of the mineral for the adsorption systems of GB and GD on the octahedral surface of dickite are about -16 and -15 kcal/mol, respectively. In the case of adsorption on the tetrahedral surface, the interaction energies of adsorption systems with GB and GD are about -7.0 and -9.0 kcal/mol, respectively.