Infrared Spectra of Carbon Monoxide in Zeolites

The infrared spectra of ¹²C¹⁶O and of a mixture of ¹²C¹⁶O and ¹³C¹⁶O in the zeolite NaA show bands that have been assigned as $\mathrm{R‐,\; Q‐}$ , and P‐branch maxima of a relatively free rotor. This conclusion depends, partly, on small but experimentally distinguishable differences in the isotope shift of the bands of a pure vibrator and those of a vibrotor. The spectrum of carbon monoxide in the zeolite NaX is similar to that in NaA. However, in CaA, and in CaX, where it is known that CO is more strongly adsorbed than in the sodium zeolites, the bands have a different structure, corresponding to a strongly hindered rotor.