**Interaction of Small Molecules within Metal Organic Frameworks Studied by** *In Situ* **Vibrational Spectroscopy**

Kui Tan and Yves Jean Chabal

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64906

#### **Abstract**

Molecular-level characterization of interaction between small gases and metal organic frameworks (MOFs) is crucial to elucidate the adsorption mechanism and establish the relationship between the structure and chemical features of MOFs with observed adsorptive properties, which ultimately guide the new structure design and synthesis for enhanced functional performance. Among different techniques, vibrational spectroscopy (infrared and Raman), which provides fingerprint of chemical bonds by their vibrational spectra, is one of the most powerful tools to study adsorbate-adsorb‐ ent interaction and give rich detailed information for molecular behaviors inside MOFs pores. This chapter reviews a number of exemplary works utilizing vibrational spectroscopy to study the interaction of small molecules with metal organic frame‐ works.

**Keywords:** interaction, small molecules, metal organic frameworks (MOFs), infrared (IR), Raman spectroscopy, vdW-DF calculation

## **1. Introduction**

In the past decade, metal organic frameworks (MOFs) have become one of the fastest growing new fields in chemistry. Tremendous advances has been made in synthesis for new struc‐ tures (>20,000 reported), structure determination or postmodification, and exploration of potential application in different fields such as gas storage and separation (H2, CO2, N2, and CO2), drug delivery, sensing, luminescence, and catalysis based on adsorption. The mecha‐ nistic understanding of the interaction of the molecules with MOFs is critical for the rational design of new MOFs with desired properties and accurate assessment of functional perform‐

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ance in practical applications. Traditional characterization methods for MOFs materials have relied mainly on physical measurements, such as X-ray diffraction, thermogravimetric, gas adsorption isotherm, and breakthrough analysis. These techniques are powerful in deriving some critical parameters, such as crystal structures, chemical composition, thermal stability, adsorptive uptake, enthalpy, and selectivity, for assessing adsorption properties; however, mechanistic information about the local bonding sites, adsorption geometry, and guest-host, guest-guest cooperative, or competitive interaction is particularly difficult to derive. Experi‐ mental methods currently employed by the community to analyze how the molecules interact with a framework include infrared and Raman spectroscopy, X-ray (neutron) diffraction, and inelastic neutron scattering.While allthese techniques have been shown to be usefulto identify the binding sites of the MOFs toward the small molecules, vibration spectroscopy, i.e., infrared and Raman spectroscopy is particularly sensitive to probe the local interaction between guest molecules and the surface of metal organic frameworks. These two spectroscopic techniques provide complementary information about the nature of interaction, bonding configura‐ tions, intermolecular attraction, or repulsion through their vibrational spectra. Furthermore, they require lower capital cost and have greater accessibility of the instrumentation, which is easily modified for *in situ* measurements in a wide range of temperatures and pressures.

In this chapter, the recent progress of infrared and Raman spectroscopy studies on the underlying interactions that govern adsorption behaviors of small molecules, i.e., H2, CO2, H2O, O2, CO, NO, H2S, SO2, in different MOFs materials is discussed and summarized. In most cases for nonreactive molecules, such as H2 and CO2, van der Waals forces dominate the interaction between the guest molecules and the building units of the MOFs. In some cases, chemical reaction involving electron transfer occurs upon adsorption of reactive molecules, e.g., H2O, leading to a significant modification of MOFs crystalline structure. Combined with calculation, especially the recent successful effort to include van der Waals forces, selfconsistently in DFT(Density functional theory) in the form of a van der Waals density func‐ tional, molecular weak physical interactions within MOFs materials are accurately described and experimental data can be well interpreted and rationalized.
