Cation-carbon stretching vibration of adducts formed upon CO adsorption on alkaline zeolites

M+... CO adducts (M = Li, Na, K, Rb, Cs) formed upon CO adsorption on alkali-metal exchanged zeolites show IR spectra characterized by a main (cation specific) IR absorption band which appears in the range 2150-2180 cm(-1), and which is due to the fundamental C-O stretching mode However, a (nu(CO)). weaker band is often observed some 90-140 cm(-1) higher than the main band. This weaker band can be assigned to the combination mode where represents the frequency of nu(CO) + nu(MC) nu(MC) the cation-carbon bond vibration. This assignment is supported by direct observation of the corresponding band (at 139 cm(-1)) for CO adsorbed on Na-Y, which was detected by using far IR radiation from a synchrotron source. Observation of the combination mode is relevant to zeolite characterization by IR spectroscopy, since the characteristic cation-carbon stretching vibration is very sensitive to the specific cation present in the zeolite.