THEORETICAL-ANALYSIS OF CO2 ADDUCTS ON THE NATIVE E'' CENTER IN ION-BOMBARDED POROUS SILICA

Density functional calculations have been performed on molecular model s of silica defects and their CO2 adducts. The results are consistent with a previous experimental infrared characterization, which was inte rpreted in terms of CO2 addition to the SiO2 skeleton at the diradical silicon defect produced by argon bombardment. According to this inter pretation, the addition of CO2 takes place first via the formation of a carboxylate group which, after annealing, evolves to an ester specie s and then to a carboxylic acid after exposure to wet air. The geometr ies and thermodynamical stabilities of the proposed species and other possible adducts have been evaluated. The calculated carbonyl stretchi ng frequencies are in good agreement with the experimental values and confirm the validity of the previous interpretation, adding further de tails and clarifying a few doubtful assignments.