Use of pyridine CH(D) vibrations for the study of Lewis acidity of metal oxides

C–H and C–D stretching vibrations of h5- and d5-pyridine adsorbed on various metal oxides have been investigated by infrared spectroscopy. Experimental spectra were further compared with density functional (DFT) calculations of the interaction of pyridine with cluster models. Both νCH(D) frequencies and intensities are sensitive to the adsorption mode and allow one to distinguish between H-bonded, protonated and coordinated species. In the case of coordinated species, an increase in ν(CH) frequencies is observed and it could be correlated with the ν8a and ν19b frequency shifts which are commonly used to assess acid strength of metal oxides. It is found that the study of ν(CH) modes is more reliable than that of the ν8a and ν19b modes to distinguish H-bonded from weakly coordinated species. An example of such an application is given for the cases of CeO2, MgO and CaO.