Quantum-chemical study of the adsorption of DMMP and sarin on γ-Al2O3

Ab initio calculations, using density functional theory with the B3LYP functional, have been applied to the adsorption of the chemical warfare agent simulant dimethyl methylphosphonate (DMMP) and the corresponding real agent Sarin on gamma-Al2O3. The goals are to determine the accuracy with which the adsorbed molecules (for which experimental data are available) can be modeled and to conduct a "side-by-side" comparison of the bonding of these species to gamma-Al2O3. Free-standing Al8O12 and Al20O30 clusters give reasonable descriptions of the adsorbate structure and properties, and the results are not strongly dependent on cluster size or basis-set quality. For either molecule, the energetically favorable mode of adsorption is Al-OP dative-bond formation, in agreement with experiment. Results for the physisorption of H2O are compared to those reported for a two-dimensionally periodic slab in order to test the reliability of the free-standing cluster model. The adsorption energy of DMMP on the Al20O30 cluster (-57.5 kcal/mol at the 6-311G(df) level) is greater than that of Sarin (-49.2 kcal/mol). The infrared-active normal-mode frequencies for free DMMP and Sarin have been used to verify the reported mode assignments for these species. For the adsorbed molecules, the nu(PO) stretch shows a red-shift (relative to the gas phase) of similar to 60 cm(-1) (observed) vs about 84 cm(-1) (calculated). The calculated shifts for other modes are much smaller and generally agree with experiment.