Figure 4. IR difference spectra of HypCD_MC that show the effect of ¹²CO, ¹³CO, and O₂.

(Fig. 4A) Binding of external ¹²CO (¹³CO) produces a new peak, 2038 cm⁻¹ (2006 cm⁻¹) and induces a shift of the original Fe–CO peak from 1927 to 1968 cm⁻¹ (1960 cm⁻¹). Note these spectra do not give rise to difference signals in the CN⁻ region. (Fig. 4B) A strong negative contribution at 1927 cm⁻¹ is observed. Double difference spectra depict on the influence of O₂ on CO–soaked sample (CO>O₂) and vice versa (O₂>CO). No additional difference bands are observed. Note these spectra do not give rise to difference signals in the CN⁻ region.