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 = =SiOH, =Al, and Mg-3c) complexes on oxides are equal within 5 cm(-1) to the 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), Mg4O4, and Mg6O6) and CD3CN . Y' (Y' = Si(OH)(4), Al(OH)(3), and Mg4O4) molecular models. This result allows to conclude that the chemical interactions responsible for these frequency shifts of CO and CD3CN on oxides and in the molecular complexes are nearly identical. It gives strength to the use of the molecular approach for the calculation of intramolecular frequency shifts of probe molecules on oxides. The Delta nu(CO) shifts of CO on Mg-3c and Mg-4c sites of magnesium oxide calculated with the Mg4O4 and Mg6O6 molecular models are in significantly better agreement with the experimental data than those obtained using both neutral and charged cluster models of the same size embedded into +/-2.0 point-charge Madelung potentials. Hence, in computing the CO frequency shifts on MgO, these simplified embedding procedures worsen the cluster models compared with their ''bare'' molecular analogs.