A CHECK OF QUANTUM-CHEMICAL MOLECULAR-MODELS OF ADSORPTION ON OXIDES AGAINST EXPERIMENTAL INFRARED DATA

The experimental Delta nu(CO) and Delta nu(CN) shifts of CO . X (X = = POH, =SiOH, =SiO(H)Al=, =Al, Mg-3c, and Mg-4c) and CD3CN . Y (Y = =SiO H, =Al, and Mg-3c) complexes on oxides are equal within 5 cm(-1) to th e calculated SCF 6-31G Delta nu(CO) and Delta nu(CN) shifts of their CO . X' (X' = OP(OH)(3), Si(OH)(4), (HO)(3)SiOHAl(OH)(3), Al(OH)(3), M g4O4, and Mg6O6) and CD3CN . Y' (Y' = Si(OH)(4), Al(OH)(3), and Mg4O4) molecular models. This result allows to conclude that the chemical in teractions responsible for these frequency shifts of CO and CD3CN on o xides and in the molecular complexes are nearly identical. It gives st rength to the use of the molecular approach for the calculation of int ramolecular frequency shifts of probe molecules on oxides. The Delta n u(CO) shifts of CO on Mg-3c and Mg-4c sites of magnesium oxide calcula ted with the Mg4O4 and Mg6O6 molecular models are in significantly bet ter agreement with the experimental data than those obtained using bot h neutral and charged cluster models of the same size embedded into +/ -2.0 point-charge Madelung potentials. Hence, in computing the CO freq uency shifts on MgO, these simplified embedding procedures worsen the cluster models compared with their ''bare'' molecular analogs.