**3. Conclusion**

Vibrational spectroscopy has been proved to be the very informative technique to investigate the interaction of small gas molecules with metal organic frameworks. By examining subtle changes in the spectra of both adsorbate and adsorbent, insightful details regarding the adsorption mechanism are revealed. With the help of theoretical calculation, which provides direct access to many properties of the system, the experimental models are validated and a complete understanding of the adsorption behaviors can be derived.

For H2, although free molecule is IR inactive, the stretching mode is activated and becomes observable once the molecule is polarized by binding to the surface. A wealth of information for the interaction details, i.e., binding site and geometry, interaction potential can be extracted by analyzing the peak position, intensity, and width.

For CO<sup>2</sup> molecules, both the perturbation of stretching and bending mode convey important information for the nature of interaction. For physical adsorption with lower binding energy (<50–60 kJ/mol), the stretching mode suffers a small shift (<15 cm−1) compare to gas phase value and the bending mode is spitted due to the loss of degeneracy. If the molecules are chemically adsorbed with a high adsorption heat over 60–70 kJ/mol, IR adsorption features of new species such as carbamate can be observed. The structural modifications for functional groups are reflected by tracking the spectroscopic signatures.

For the reactive molecules such as H2O, O2, H2S, SO, and NO adsorbing into MOFs, the crystalline structure is strongly modified and even become degraded. By examining the difference spectra before and after adsorption, the weak point of the complicated MOFs structure can be identified and reaction pathway can be also unveiled, which is crucial to design robust structure.

Finally, infrared spectroscopy provides an unique advantage to study the adsorption behav‐ iors of mixture components since the vibrational modes of different molecules usually can be well distinguished in the infrared spectra. The occupation of actual adsorption sites for mixtures can be measured as a function of parameters such as time, temperature, and partial pressure. Recent works in measuring CO2 competition with a variety of molecules, e.g., H2O, NH3, SO2, NO, and NO2 in MOF-74 show kinetics for exchange process is an important parameter which needs to be taken into account for coadsorption and separation process. It also underscores the need of combined studies, using spectroscopic methods and *ab initio* simulations to uncover the atomistic interactions of small molecules in MOFs that directly influence coadsorption.
