**2. Interaction of small molecules with MOFs**

### **2.1. Energy carrier H2**

H2 molecule is IR inactive as other homonuclear diatomic molecules due to their lack of dipole moment; however, once the molecule interacts with the MOF, it undergoes a perturbation that polarizes the originally symmetric molecule and makes it weakly IR active. This perturbation is usually accompanied by red shift of the H─H stretching modes, located at 4161 and 4155 cm−1 for para and ortho H2, respectively. Nijem's measurement on different type of prototypical MOFs suggest that magnitudes of the H2 stretching frequency shifts, intensities, and line widths contains important information about the nature of the H2 interaction in the pores and depend on the structure and chemical nature of the MOF hosts [1].

By examining several prototypes of metal organic framework materials such as M(bdc) (ted)0.5 (bdc = 1, 4-benzenedicarboxylate; ted = triethylenediamine), M(bodc)(ted)0.5 (M = Ni, Co; bodc = bicyclo[2. 2. 2]octane-1, 4-dicarboxylate), M3(HCOO)6 (M = Ni, Mn, Co, HCOO = formate), and Zn2(bpdc)2(bpee), where bpdc = 4,4′-biphenyl dicarboxylate and bpee = 1,2 bis(4-pyridyl)ethylene [1], it is concluded (see **Table 1**) that IR shifts are dominated by the environment (organic ligand, metal center, and structure) rather than the strength of the in‐ teraction. For instance, the organic ligands with π = electrons such as benzene rings cause the frequency of H2 shift more than the ligand without π = electrons, e.g., H2 bands are more shifted (~−3 cm−1) for Ni(bdc)(ted)0.5 than Ni(bodc)(ted)0.5 even though Ni(bodc)(ted)0.5 has a higher binding energy. The same observation was also found by comparing the IR shift (−38 cm−1) of hydrogen molecules in Zn(bdc)(ted)0.5 with that (−30 cm−1) in MOF-74 (also called M2(dobdc), M = metal ions; dobdc4−= 2,5-dioxidobenzene-1,4-dicarboxylate), a structure con‐ taining unsaturated metal center Zn2+ with a higher binding energy (10 kJ/mol).


Reprinted with permission from [1]. Copyright (2010) American Chemical Society.

**Table 1.** Comparison between the different properties of the different MOFs.

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

H2 molecule is IR inactive as other homonuclear diatomic molecules due to their lack of dipole moment; however, once the molecule interacts with the MOF, it undergoes a perturbation that polarizes the originally symmetric molecule and makes it weakly IR active. This perturbation is usually accompanied by red shift of the H─H stretching modes, located at 4161 and 4155 cm−1 for para and ortho H2, respectively. Nijem's measurement on different type of prototypical MOFs suggest that magnitudes of the H2 stretching frequency shifts, intensities, and line widths contains important information about the nature of the H2 interaction in the pores and

and experimental data can be well interpreted and rationalized.

depend on the structure and chemical nature of the MOF hosts [1].

**2. Interaction of small molecules with MOFs**

**2.1. Energy carrier H2**

202 Metal-Organic Frameworks

**Figure 1.** Hydrogen adsorption sites in Zn(bdc)(ted)0.5 (left) and Zn2(bpdc)2(bpee) (right). The interaction lines between one of the H atoms and carbon atoms in four different ligands are illustrated. Reprinted with permission from [1]. Copyright (2010) American Chemical Society.

Integrated intensity of the H2 stretching modes is a sensitive measure of the number and symmetry of the sites and local interaction between H2 and organic ligand. Asymmetric site with multiple interaction points produced larger induced dipole moment and the correspond‐ ing IR cross-section is higher [1]. The symmetric sites lead to reduced dynamic dipole moment and lower the IR band intensity. **Figure 1** compares the adsorption site of H2 in M(bdc) (ted)0.5 and Zn2(bpdc)2(bpee): H2 interacts with several benzene rings in Zn2(bpdc)2(bpee) and the adsorption site is very asymmetric. Furthermore, the delocalized electrons in the double benzene rings in Zn2(bpdc)2(bpee) is more easily polarized by adsorbed H2 than that in the single benzene ring in Zn(bdc)(ted)0.5. All these factors cause IR intensity of H2 adsorbed in Zn2(bpdc)2(bpee) almost a magnitude higher than that of H2 adsorbed in M(bdc)(ted)0.5.

It is worth to note that IR is not only sensitive to host-gust interaction but also capable to detect molecular interactions within confined nanopores. In MOF-74 with unsaturated metal center, a small shift (−30 cm−1 with respect to the unperturbed molecules) is observed in the low loading regime when H2 is dominantly adsorbed on the metal site [2]. Additional ~−32 cm−1 IR shift and a large variation in dipole moment are observed once the neighboring oxygen site was occupied with H2 molecule to form a "pair" with H2 molecules on the metal site. Since large variation of dynamic dipole moment take place as a function of loading, due to the interaction among the adsorbed molecules and therefore the integrated areas of IR bands do not always correlate with the amount of molecules adsorbed. Cautions must be taken when using variable temperature IR to measure the absorbance of molecular hydrogen bands and estimate the adsorption energy [3].
