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