Infrared Spectra of Carbon Monoxide in an Argon Matrix

The infrared absorption spectrum of carbon monoxide in an argon matrix shows prominent bands at 2148.8 and 2138.0 cm—1 with half-widths, respectively, 1.5 cm—1 and 3.5 cm—1. The relative intensities of these bands are extremely dependent upon a variety of experimental conditions, including sample concentration, window temperature, and deposition rates. The variability shows that CO isolated in argon absorbs at 2148.8 cm—1 and that it does not rotate. The lower frequency absorption is due to aggregates. The frequency shift of the argon-isolated CO absorption relative to that of gaseous CO shows that carbon monoxide fits tightly in the argon lattice. In contrast, the CO aggregates absorb at lower frequency than gaseous carbon monoxide. Isotopic studies using 13C16O reveal vibrational coupling of 1.1 cm—1 between isotopically identical molecules in an aggregate. An increase in matrix cavity diameter would be needed if a stationary CO molecule begins to rotate in argon. This ``site expansion,'' which could be as much as 0.4 Å, may be an important factor inhibiting rotation.