Adsorption of CO₂ on MIL-53(Al) and NH₂-MIL-53(Al) has been studied by Fourier transform infrared (FTIR) spectroscopy at different temperatures and equilibrium pressures. For better interpretation of the spectra ¹³CO₂ was also utilized. It is established that with both samples at low coverages CO₂ forms O-bonded complexes with the structural OH groups (OH···O¹²CO). These species are characterized by ν₃(¹²CO₂) at 2337–2338 cm^–1 and two ν₂(¹²CO₂) modes around 662 and 650 cm^–1. Simultaneously, the ν(OH) modes of the hydroxyl groups are red-shifted, while the δ(OH) modes are blue-shifted. At higher coverages (OH···O¹²CO)₂ dimeric species are formed and this leads to a decrease of the ν₃(CO₂) frequency by 2–4 cm^–1. This change is due to vibrational interaction as proven by the observation that the frequency remains practically unaffected for (OH···O¹²CO) (OH···O¹³CO) dimeric species. Interaction between dimers leads to additional slight decrease of the value of ν₃(CO₂). In parallel with the CO₂ adsorption a partial transformation of the material from large-pore to narrow-pore form occurs. Far before CO₂ interacts with all hydroxyl groups, polymeric CO₂ species are produced within the MIL-53(Al) sample. They are characterized by a split ν₃(CO₂) mode with a pronounced component at 2340 cm^–1. The formation of these species involves some of the dimers and is accompanied by a reopening of the MIL-53 structure. Analysis of the shift of the OH modes led to the conclusion that the polymeric moiety interacts strongly with one OH group and more weakly with several other hydroxyls. No polymeric species were observed with the NH₂-MIL-53(Al) sample which is associated with the more stable narrow-pore structure of this material. However, evidence of interaction between CO₂ and the hydroxyls H-bonded to amino groups was found.

The state of the accessible copper ions in a Cu-basolite C300 sample (copper benzene-1,3,5-tricarboxylate) was studied by CO as an IR probe molecule. Low temperature CO adsorption on a sample activated at 473 K results in a series of carbonyl bands. Three intense sharp bands at 2141, 2128 and 2123 cm⁻¹ are very sensitive to the CO equilibrium pressure and are associated with CO polarized by the aromatic ring. Three bands, at ca. 2192, 2178 and 2148 cm⁻¹ are assigned to three different kinds of monocarbonyl species. At high CO coverage, the principal carbonyls (2178 cm⁻¹) are converted into polycarbonyls. A band at 2125 cm⁻¹ (resistant to evacuation) is associated with Cu⁺ ions on small CuO particles. Experiments on ¹²CO– ¹³CO adsorption revealed lack of vibrational coupling and also showed that the bands at 2192 and 2148 cm⁻¹, although changing almost in concert, were due to individual species. Sample activated at 373 K does not contain copper oxide, which indicates that activation at higher temperatures, 473 K, leads to some irreversible changes.

The combined use of CO and N₂as IR probe molecules allows a precise measuring of the acidity of OH groups that are preliminary involved in weak H-bonding.

FTIR spectra of ¹² CO ₂ and ¹² CO ₂ + ¹³ CO ₂ mixtures adsorbed on MIL-53(Al) reveal the formation of highly symmetric dimeric (CO ₂ ) ₂ species connected to two structural OH groups.