Carbon monoxide stretching frequency

A variety of instmments are available to analyze carbon monoxide in gas streams from 1 ppm to 90%. One group of analyzers determines the concentration of carbon monoxide by measuring the intensity of its infrared stretching frequency at 2143 cm . Another group measures the oxidation of carbon monoxide to carbon dioxide electrochemically. Such instmments are generally lightweight and weU suited to appHcations requiring portable analyzers. Many analyzers are equipped with alarms and serve as work area monitors. 

This simple picture of bonding is convenient to use, and often completely acceptable. However, it does lack sophistication and may not be used to explain some of the subtleties of these systems. One obvious point in this regard concerns infrared spectral data. Coordination of carbon monoxide to a metal invariably leads to a lower carbonyl stretching frequency (vco). implying a lower CO bond order as predicted. However, the values for vcn may be considerably higher for metal complexes of an isocyanide than are the values for the ligand itself. The valence-bond picture cannot rationalize... 

Adsorption of carbon monoxide takes place all over the surface and there is distinct evidence that, at least on nickel, the CO stretching frequency depends upon the coordination number of the nickel atom to which it is attached. Hence, the adsorption of carbon monoxide yields information about the relative numbers of surface atoms with different coordination numbers. This information, howrever, is at best merely of a semiquantitative nature. Steric effects also play a role, as is evidenced by the fact that the subcarbonyl species can be formed only on nickel atoms with a lowr coordination number. 

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-0 stretch frequency is very informative about the direct environment of the molecule. The metal-carbon bond, however, falling at frequencies between 300 and 500 cm1, is more difficult to measure with infrared spectroscopy. First, its detection requires special optical parts made of Csl, but even with suitable equipment the peak may be invisible because of absorption by the catalyst support. In reflection experiments on single crystal surfaces the metal-carbon peak is difficult to obtain because of the low sensitivity of RAIRS at low frequencies [12,13], EELS, on the other hand, has no difficulty in detecting the metal-carbon bond, as we shall see later on. 

Rh-Rh vector may donate electron density to the empty Rh-Rh (T -orbital and/or overlap through back-bonding with either of the fiUed n MOs [103]. Infrared measurements of Rh2f4-CO adducts demonstrate clearly a red shift in the C=0 stretching frequency relative to imhgated carbon monoxide [104]. These results are consistent with electron delocalization from the Rh-Rh n into the low-lying Similarly, back-... 

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