# Hierarchical Structure and Molecular Dynamics of Metal-Organic Framework as Characterized by Solid State NMR

Authors: Wei Chen; Yi-nan Wu; Fengting Li
Journal: Journal of Chemistry
 (2016)

---

## Abstract

Metal-organic framework (MOF) stands out as a promising material with great potential in application areas, such as gas separation and catalysis, due to its extraordinary properties. In order to fully characterize the structure of MOFs, especially those without single crystal, Solid State NMR (SSNMR) is an indispensable tool. As a complimentary analytical technique to X-ray diffraction, SSNMR could provide detailed atomic level structure information. Meanwhile, SSNMR can characterize molecular dynamics over a wide dynamics range. In this review, selected applications of SSNMR on various MOFs are summarized and discussed.


---

## Body

## 1. Introduction

Metal-organic framework, or MOF, consisting of metal ion center and organic ligand attracts numerous attention since 1990s [1]. As an inorganic and organic hybrid, MOF owns extraordinary properties, such as ultrahigh internal surface area, tunable internal core size and high thermal stability. The modification of the organic ligand could further provide us with an excellent platform to adjust surface properties for proper applications. Due to these excellent characteristic features, MOFs own great potential in various application areas such as gas storage (i.e., CO2, CH4, and H2) [2–4], separation [5], catalysis [6], and biomedicine [7].The macroscopic performance of MOFs is highly related to microscopic structure and dynamics. The understanding of this relationship is crucial for us to optimize current MOFs or design new MOFs. For MOF structure, although the crystal structure of many MOFs can be determined through single crystal X-ray diffraction, for numerous MOF systems, such as the porous aluminum terephthalate (MIL-53) [8], it is difficult to obtain single crystal and the structural information always need to be refined based on powder X-ray diffraction results. Therefore, other techniques that can access to molecular level atomic position are necessary to refine the existing crystal structure. For MOF dynamics, it is required to obtain molecular dynamics information of MOFs at elevated temperature or under different pressure for industrial application. For instance, in the case of the storage of CO2, it is necessary to obtain direct information about the interaction between CO2 and MOFs and CO2 adsorption dynamics [9].Solid State NMR (SSNMR) technique stands out as an analytical technique to characterize both local structure and dynamics and the interplay between them [10–12]. SSNMR is able to probe internuclear distance based on various interactions such as dipolar coupling, quadrupolar interaction, and chemical shift anisotropy (CSA). As a result, SSNMR could provide detailed local packing structure and short range ordering information, as a complimentary technique to X-ray diffraction [13]. This work provides a brief review of the application of SSNMR in characterizing both structure and dynamics of MOFs with selected examples.


## 2. Investigation of MOF Structure by SSNMR

### 2.1. Identification of Complex State of Central Metal Ion

The local environment of central metal ion is the crucial information to determine the MOFs’ structure. Also, processes, such as absorption of gas and hydration/dehydration, could change the local structure of central metal ion. This leads to the change of metal ion NMR spectroscopy. Therefore, SSNMR of various metal ions will be first presented.Table1 lists selected NMR active metal ions with basic NMR properties including spin numberI, natural abundance, receptivity, Larmor frequencyv0 (at magnetic field strength B0 = 11.7 T), and quadrupole momentQ. And detailed description of these properties of each nucleus and selected examples of application are discussed as follows.Table 1
Selected metal ions with different NMR propertiesb.
Isotope SpinI Natural abundance(%) R e c e p t i v i t y a ν 0 at B0=11.7 T (MHz) Quadrupole momentQ/mb 27Al 5/2 100 0.207 130 146.6 45Sc 7/2 100 0.302 121 −220 69Ga 3/2 60 0.0419 120 171 71Ga 3/2 40 0.0571 152 107 25Mg 5/2 10 0.000268 31 199.4 47Ti 5/2 7.44 0.000156 28 302 49Ti 7/2 5.41 0.000205 28 247 91Zr 5/2 11.22 0.00107 47 176 67Zn 5/2 4.1 0.000118 31 150 a: relative to1H = 1.00; b: all data are adapted from https://www.webelements.com.27 Al SSNMR. 27Al SSNMR is one of the most studied metal ions due to its high receptivity (relative to 1H) and moderate quadrupolar interaction. Similar to other metal ions with spin I>1/2, 27Al (5/2) SSNMR spectrum is characterized by quadrupolar coupling constant (CQ) and asymmetry parameter η representing the electric field gradient (EFG) of metal ion. Latter reflects the local environment close to the metal ion and can be used to determine the crystal structure or observe the structural change. 27Al SSNMR has been widely used to characterize inorganic materials such as cements [14–17]. Following is selected literature related to Al based MOF systems.The aluminum 1,4-benzenedicarboxylate (BDC) based MIL-53 is one of the attractive MOFs which has great potential in gas separation and hydrogen storage due to its very large breathe effect [18]. As shown in Figure 1, there are three different forms of MIL-53 (Al) under different treatments: the as-synthesized MIL-53 (Al) (MIL-53as (Al)) contains two BDC molecules in each channel; when MIL-53as (Al) is annealed at high temperature (~330°C) for three days, the MIL-53ht (Al) will be obtained without any free BDC molecules in the channel; after cooling to ambient temperature, the MIL-53lt (Al) is formed with water molecule absorbed inside [8]. Loiseau et al. investigated the MIL-53 (Al) structure changes through 27Al SSNMR as shown in Figure 2 [8]. Figure 2(a) shows 27Al SSNMR spectra of MIL-53 (Al) in different phases. The MIL-53as (Al) displays a broad shoulder originating from the amorphous Al(OH)3, and, after calcination, 27Al SSNMR spectrum becomes shape suggesting more homogeneous systems. Due to large open space created in MIL-53ht (Al), small molecules such as water are able to be absorbed. Because of well distinct NMR spectra of dehydrated and hydrated MIL-53 (Al), it is able to track the structural transition of MIL-53 (Al) during hydration process. After absorption of guest water molecule, a significant broad 27Al SSNMR spectrum was obtained representing stronger quadrupolar coupling strength. Figure 2(b) shows the in situ observation of the hydration process of MIL-53 (Al), and the hydration ratio is increased from bottom to top as 0%, 30%, 50%, and finally 100%. A continuous increase of CQ could be obtained suggesting the break of local symmetry.Figure 1
3D structure of MIL-53 (Al) in three different states: (a) as-synthesized MIL-53(Al) or MIL-53as (Al), where two disordered 1-4 benzenedicarboxylic acid molecules are inserted in the channel; (b) calcined MIL-53ht (Al), where the channel is empty; (c) water saturated MIL-53lt (Al), where one water molecular locates in the channel. Reprinted with permission from Ref. [8]. Copyright 2004 John Wiley and Sons.
(a) (b) (c)Figure 2
27Al SSNMR spectra of MIL-53 (Al) in (a) different phases and (b) different hydration content. Reprinted with permission from Ref. [8]. Copyright 2004 John Wiley and Sons. The dot line represents the simulated 27Al NMR line shape and ∗ refers to residual Al(OH)3.
(a) (b)Later on, Jiang et al. changed the organic linker BDC into 2-aminobenzene-1,4-dicarboxylic acid (ABDC) and found that the quadrupolar coupling strengthCQ decreases ~1 MHz indicating more symmetric local structure and more homogeneous charge distribution [19]. They also investigated the dehydration/rehydration process of this MOF. The evolution of 27Al SSNMR spectra upon hydration process is similar to above results. And the broadening of 27Al NMR resonance line is attributed to the strong interaction formed by guest water molecule (water) and host MOF, especially the hydrogen bonding formed between water and carboxylate.MIL-118, Al2(OH)2[C10O8H2], is a new aluminum pyromellitate developed by Volkringer et al. [20]. The as-synthesized MIL-118A, Al2(OH)2(H2O)2[C10O8H2], can be transformed into MIL-118B, Al2(OH)2[C10O8H2], upon heating. The rehydration of MIL-118B leads to MIL-118C, Al2(OH)2[C10O8H2]·2.75H2O. 27Al SSNMR was used to refined the crystal structure of these three phases, and it was found that MIL-118A and B display single 27Al signal, while MIL-118C shows two resonance lines. This suggests Al is magnetically equivalent in MIL-118 A/B and has two inequivalent sites in MIL-118C, which is consistent with X-ray diffraction results. The overlap of NMR signal of different Al sites induces dramatic difficulties in peak assignment and determination of the composition ratio of different Al. Due to well resolved NMR signal, the multiquantum (MQ) NMR is applied to obtain quadrupolar parameters and isotropic chemical shift of different sites [21–23]. Figure 3 presents the contour plot of 3Q NMR spectrum of MIL-118A. The multiquantum coherences in one dimension are correlated with central transition (CT) coherence in another dimension. 27Al NMR signals of different sites are well resolved, and relative composition ratio of different species can also be obtained.Figure 3
Contour plots of27Al 3QMAS NMR of MIL-118A. Reprinted with permission from Ref. [20]. Copyright 2009 American Chemical Society.45 Sc SSNMR. Scandium based materials own great potential in the application of catalyst, such as scandium(III) trifluoromethanesulfonate [Sc(OTf)3], which can be used to catalyze the carbon-carbon bond-forming reaction in aqueous media [24, 25]. Due to its good receptivity and moderate quadrupolar interactions as listed in Table 1, 45Sc has good NMR properties. Rossini et al. characterized the Sc(OTf)3 structure through the combination of 45Sc SSNMR and powder X-ray diffraction [26, 27]. For Sc(OTf)3 it is difficult to obtain single crystal causing dramatic difficulties in structural determination. They chose Sc(OAc)3 as a reference sample due to the well determined crystal structure. Meanwhile, the 45Sc SSNMR spectra of both Sc(OTf)3 and Sc(OAc)3 are quite similar, and the obtained EFG tensor parameters are almost identical. This suggests that the Sc coordination states are almost the same in both samples. On the basis of the 3D structure of Sc(OAc)3, the crystal structure of Sc(OTf)3 could be determined through the powder X-ray diffraction pattern.Besides catalysts, scandium based MOF can also be used in gas separation due to the porous structure. Miller et al. characterized the structure difference between scandium terephthalate, Sc2(BDC)3, and its derivatives, which is able to absorb CO2, through 45Sc SSNMR [28]. 45Sc SSNMR spectrum of Sc2(BDC)3 displays a characteristic second-order quadrupolar NMR line shape representing a single site for Sc. However, 45Sc SSNMR spectra of its derivatives, amino- and nitro-BDC, display featureless resonance line indicating statistical disorder of the orientation of the functionalized derivatives.69/71 Ga SSNMR. Gallium owns two NMR active nucleuses: 69Ga and 71Ga as listed in Table 1. As compared with 71Ga, 69Ga has larger CQ, which displays more broadened line shapes. Therefore, 71Ga is favored in SSNMR experiment. Compared with 27Al, 71Ga owns larger second-order quadrupolar splitting (wQ=3CQ/4I(2I-1)) due to its low spin number (I=3/2). This leads to broader NMR resonance lineshape. Meanwhile, the natural abundance of 71Ga is 40%, while that of 27Al is 100%. Therefore, either NMR sensitivity or resolution of 71Ga is lower than those of 27Al. In order to overcome these deficiencies, several software (new pulse sequence) and hardware (fast magic angle spinning, MAS) techniques are utilized. Summary of these techniques can be found in relevant reviews [29].Hajjar et al. investigated71Ga coordination environment in MIL-120 and MIL-124 through CT (central transition) MAS [30] NMR in conventional NMR probe [31]. In combination of the NMR simulation SIMPSON [32] and 2D slow-CTMAS results, EFG parameters of different crystallographic Ga sites are determined; there are two distinct Ga sites in MIL-120 and one in MIL-124 with remaining site undetectable due to fast molecular motion. This paper shows that classical NMR probe without ultrahigh magnetic field and ultrahigh spinning rate can also be used to detect 71Ga NMR signal and extract useful structural information. Ash and Grandinetti systematically investigated 69/71Ga SSNMR in different coordination state [33]. With the aid of ultrafast MAS spinning rate (30 kHz), the whole spectra of 69/71Ga NMR are able to be obtained. The isotropic chemical shift of 69/71Ga is found to be highly dependent on the coordination state: four coordinates lead to 50 ppm and six coordinates result in 225 ppm. Volkringer et al. studied gallium trimesate (MIL-96) through 71Ga SSNMR in high magnetic field [34]. 71Ga SSNMR spectrum in the magnetic field strengthB0 = 14.1 T shows a featureless single resonance line. However, under the higherB0 = 17.6 T, two distinct 71Ga signals can be observed. This enable us to extract the isotropic chemical shift and EFG parameters to identify different Ga sites in MOF system.25 Mg/ 47/49 Ti/ 91 Zr/ 67 Zn SSNMR. Despite above nucleuses, other frequently used metal ions in MOFs such as 25Mg, 47/49Ti, 91Zr, and 67Zn in MOFs experience more severe problems in NMR signal and sensitivity. As shown in Table 1, the natural abundance of these nucleuses is quite low (<12%). This requires more NMR experiment time to get a NMR spectrum with an acceptable signal-to-noise ratio. Moreover, the low gyromagnetic ratio γ (with corresponding Larmor frequency <50 MHz at 11.7 T) is beyond classical NMR probe sensitive range and large quadrupolar interactions (Q>150 MB) leads to broad quadrupolar lineshape. These unfavorable factors result in the low receptivity (<0.001). Therefore, both NMR sensitivity and resolution are extremely poor. To overcome these problems, ultrahigh magnetic field, ultrafast MAS spinning rate (≫10 kHz), and sensitivity enhancement techniques are necessary.Huang’s group used25Mg SSNMR to investigate the magnesium based MOF systems at high magnetic field (21.1 T) [35, 36]. The dehydration and rehydration process of CPO-27-Mg was in situ observed through 25Mg SSNMR [35]. The dehydration process leads to broadened 25Mg SSNMR lineshape indicating that the system becomes more disordered. And the dehydrated MOF presents featureless 25Mg NMR signal, which comes from the large CQ resulted from the distorted symmetry.In literature,47,49Ti SSNMR was normally obtained through sensitivity enhancement pulse sequences, such as fast amplitude-modulated pulse trains [37] or high magnetic field [16, 38, 39]. Due to almost the same Larmor frequency, 47Ti and 49Ti NMR signals always overlap with each other. Gervais et al. investigated the phase distribution of Titania nanoparticle through 47,49Ti SSNMR. Due to different isotropic chemical shift and EFG parameters for different Ti, 47,49Ti SSNMR can be used to get quantitative information of different Ti species with different local symmetry. Rossini et al. studied Titanocene Chlorides through the combination of 47,49Ti SSNMR and quantum chemical calculation [39]. In this case, 47Ti and 49Ti NMR spectra were observed separately at 21.1 T. The obtained NMR line shape together with EFG parameters and chemical shift (CS) tensors clarifies the Ti local structure and the symmetry.91Zr SSNMR was first reported in 1991 to investigate synthetic oxide materials [40]. 91Zr SSNMR line shape was found to be highly dependent on local symmetry. Zhu et al. investigated 91Zr SSNMR in two zirconium silicates AM-2 and AV-3 [41]. It was found that not only the local symmetry and distortion of the [ZrO6] but also the second and third spheres of atoms could influence NMR line shape. This shows that the quadrupolar interaction and CS tensor could also reflect surrounding environment.Sutrisno et al. studied Zn-contained MOFs (zeolitic imidazolate frameworks ZIF-8, ZIF-14, ZIF- 4, and ZIF-7) through67Zn SSNMR [42]. The 67Zn NMR line shape is determined not only by the local environment but also by the guest molecules. In combination with molecular dynamics, detailed information of the distribution and dynamics of guest molecules can be obtained. This paper, for the first time, shows the potential of 67Zn SSNMR in determining the detailed structure of Zinc based MOFs as a powerful complimentary characterization tool to X-ray diffraction.


### 2.2. Identity of Organic Linker

Despite the central metal ion, another important building block of MOFs is organic linker, or organic ligand. The organic linker is mostly constituted by carbon, proton, and oxygen.13C and 1H SSNMR are frequently used to identify the presence or absence of certain functional groups. As shown in Figure 4, 13C and 1H NMR spectra of MIL-53 in different phases display characteristic NMR line features [8]. 13C direct polarization (DP) MAS NMR spectrum could provide quantitative information of 13C of different functional groups as the integrated intensity is proportional to the number of 13C. As shown in Figure 4(a), MIL-53as (Al) displays relatively broad NMR resonance line suggesting the heterogeneity of the system. However, after thermal treatment at high temperature, a narrow and sharp 13C resonance line could be observed suggesting a highly ordered structure with high crystallinity. Further incorporation of guest molecules (i.e., water) broadens and shifts NMR resonance line indicating strong interaction between host (MIL-53) and guest (water) molecules. 13C cross polarization (CP) MAS NMR, most of the time, provides us with qualitative information about different 13C species due to the cross polarization efficiency highly relying on the interaction between observed 13C and neighboring 1H, which can be modulated by various factors, such as internuclear distance and molecular motion. As shown in Figure 4(b), when incorporated with guest molecule (water), the carbonyl carbon NMR signal almost disappears as a result of lower CP efficiency caused by the enhanced molecular motions.Figure 4
(a)13C DPMAS NMR spectrum, (b) 13C CPMAS NMR spectrum, and (c) 1H MAS NMR spectrum of MIL-53 in different phases. Reprinted with permission from Ref. [8]. Copyright 2004 John Wiley and Sons.
(a) (b) (c)1H SSNMR usually suffers from poor resolution resulting from the small chemical shift range and strong homonuclear dipolar coupling (~60 kHz). With great advances in both hardware (i.e., fast MAS > 30 kHz) and software (new pulse sequence), the NMR resolution of proton spectrum has been largely increased. Nowadays, 1H SSNMR has been widely used to study complicated systems, such as supramolecules [43–45] and biological areas [46–49]. As shown in Figure 4(c), up to three distinct proton signals can be obtained in MIL-53as (Al). Upon thermal treatment, the free carboxylic acid is completely removed, and further hydration treatment introduces the signal of water. Meanwhile, the hydroxyl group, Al-OH-Al, is directly observed throughout all three samples.Some MOFs contain paramagnetic metal center such as Cu3(BTC)2 (copper (II) benzene 1,3,5-tricarboxylate) [50]. The introduction of paramagnetic ions could largely influence the local magnetic field [51]. It will change the 1H and 13C chemical shift, broaden the NMR resonance line, and reduce the spin-lattice relaxation time [52]. Gul-E-Noor et al. found that the proton signal of water absorbed by Cu3(BTC)2 can shift downfield about 7.0 ppm [50]. For 13C SSNMR spectrum, signal of aromatic part shifts downfield of 98–92 ppm, while that of the carbonyl carbon shifts upfield of ~300 ppm. The possible origin of such difference is ascribed to closer interaction of copper and carboxylic groups.Oxygen is another frequently encountered atom in MOFs as organic carboxylate ligand is an important building block for many MOFs. Also, other functional groups such as hydroxyl (-OH-), oxygen anion (O2-) interacting with metal ions, and guest molecules (i.e., water) contain oxygen atom. However, the only NMR active isotope is 17O, whose natural abundance is 0.038%. 17O enrichment is necessary to do 17O SSNMR leading to high cost. As a result, little literature about 17O SSNMR of MOFs has been reported. He et al. reported an innovative method to prepare 17O enriched MOFs and characterize 17O of different sites through SSNMR [53]. 17O enriched MOFs are prepared through incorporating 17O atom from H2O17. The EFG parameters and chemical shift tensor information can be obtained through 17O SSNMR. Thus providing detailed structural information and identification of different oxygen contained species.


## 2.1. Identification of Complex State of Central Metal Ion

The local environment of central metal ion is the crucial information to determine the MOFs’ structure. Also, processes, such as absorption of gas and hydration/dehydration, could change the local structure of central metal ion. This leads to the change of metal ion NMR spectroscopy. Therefore, SSNMR of various metal ions will be first presented.Table1 lists selected NMR active metal ions with basic NMR properties including spin numberI, natural abundance, receptivity, Larmor frequencyv0 (at magnetic field strength B0 = 11.7 T), and quadrupole momentQ. And detailed description of these properties of each nucleus and selected examples of application are discussed as follows.Table 1
Selected metal ions with different NMR propertiesb.
Isotope SpinI Natural abundance(%) R e c e p t i v i t y a ν 0 at B0=11.7 T (MHz) Quadrupole momentQ/mb 27Al 5/2 100 0.207 130 146.6 45Sc 7/2 100 0.302 121 −220 69Ga 3/2 60 0.0419 120 171 71Ga 3/2 40 0.0571 152 107 25Mg 5/2 10 0.000268 31 199.4 47Ti 5/2 7.44 0.000156 28 302 49Ti 7/2 5.41 0.000205 28 247 91Zr 5/2 11.22 0.00107 47 176 67Zn 5/2 4.1 0.000118 31 150 a: relative to1H = 1.00; b: all data are adapted from https://www.webelements.com.27 Al SSNMR. 27Al SSNMR is one of the most studied metal ions due to its high receptivity (relative to 1H) and moderate quadrupolar interaction. Similar to other metal ions with spin I>1/2, 27Al (5/2) SSNMR spectrum is characterized by quadrupolar coupling constant (CQ) and asymmetry parameter η representing the electric field gradient (EFG) of metal ion. Latter reflects the local environment close to the metal ion and can be used to determine the crystal structure or observe the structural change. 27Al SSNMR has been widely used to characterize inorganic materials such as cements [14–17]. Following is selected literature related to Al based MOF systems.The aluminum 1,4-benzenedicarboxylate (BDC) based MIL-53 is one of the attractive MOFs which has great potential in gas separation and hydrogen storage due to its very large breathe effect [18]. As shown in Figure 1, there are three different forms of MIL-53 (Al) under different treatments: the as-synthesized MIL-53 (Al) (MIL-53as (Al)) contains two BDC molecules in each channel; when MIL-53as (Al) is annealed at high temperature (~330°C) for three days, the MIL-53ht (Al) will be obtained without any free BDC molecules in the channel; after cooling to ambient temperature, the MIL-53lt (Al) is formed with water molecule absorbed inside [8]. Loiseau et al. investigated the MIL-53 (Al) structure changes through 27Al SSNMR as shown in Figure 2 [8]. Figure 2(a) shows 27Al SSNMR spectra of MIL-53 (Al) in different phases. The MIL-53as (Al) displays a broad shoulder originating from the amorphous Al(OH)3, and, after calcination, 27Al SSNMR spectrum becomes shape suggesting more homogeneous systems. Due to large open space created in MIL-53ht (Al), small molecules such as water are able to be absorbed. Because of well distinct NMR spectra of dehydrated and hydrated MIL-53 (Al), it is able to track the structural transition of MIL-53 (Al) during hydration process. After absorption of guest water molecule, a significant broad 27Al SSNMR spectrum was obtained representing stronger quadrupolar coupling strength. Figure 2(b) shows the in situ observation of the hydration process of MIL-53 (Al), and the hydration ratio is increased from bottom to top as 0%, 30%, 50%, and finally 100%. A continuous increase of CQ could be obtained suggesting the break of local symmetry.Figure 1
3D structure of MIL-53 (Al) in three different states: (a) as-synthesized MIL-53(Al) or MIL-53as (Al), where two disordered 1-4 benzenedicarboxylic acid molecules are inserted in the channel; (b) calcined MIL-53ht (Al), where the channel is empty; (c) water saturated MIL-53lt (Al), where one water molecular locates in the channel. Reprinted with permission from Ref. [8]. Copyright 2004 John Wiley and Sons.
(a) (b) (c)Figure 2
27Al SSNMR spectra of MIL-53 (Al) in (a) different phases and (b) different hydration content. Reprinted with permission from Ref. [8]. Copyright 2004 John Wiley and Sons. The dot line represents the simulated 27Al NMR line shape and ∗ refers to residual Al(OH)3.
(a) (b)Later on, Jiang et al. changed the organic linker BDC into 2-aminobenzene-1,4-dicarboxylic acid (ABDC) and found that the quadrupolar coupling strengthCQ decreases ~1 MHz indicating more symmetric local structure and more homogeneous charge distribution [19]. They also investigated the dehydration/rehydration process of this MOF. The evolution of 27Al SSNMR spectra upon hydration process is similar to above results. And the broadening of 27Al NMR resonance line is attributed to the strong interaction formed by guest water molecule (water) and host MOF, especially the hydrogen bonding formed between water and carboxylate.MIL-118, Al2(OH)2[C10O8H2], is a new aluminum pyromellitate developed by Volkringer et al. [20]. The as-synthesized MIL-118A, Al2(OH)2(H2O)2[C10O8H2], can be transformed into MIL-118B, Al2(OH)2[C10O8H2], upon heating. The rehydration of MIL-118B leads to MIL-118C, Al2(OH)2[C10O8H2]·2.75H2O. 27Al SSNMR was used to refined the crystal structure of these three phases, and it was found that MIL-118A and B display single 27Al signal, while MIL-118C shows two resonance lines. This suggests Al is magnetically equivalent in MIL-118 A/B and has two inequivalent sites in MIL-118C, which is consistent with X-ray diffraction results. The overlap of NMR signal of different Al sites induces dramatic difficulties in peak assignment and determination of the composition ratio of different Al. Due to well resolved NMR signal, the multiquantum (MQ) NMR is applied to obtain quadrupolar parameters and isotropic chemical shift of different sites [21–23]. Figure 3 presents the contour plot of 3Q NMR spectrum of MIL-118A. The multiquantum coherences in one dimension are correlated with central transition (CT) coherence in another dimension. 27Al NMR signals of different sites are well resolved, and relative composition ratio of different species can also be obtained.Figure 3
Contour plots of27Al 3QMAS NMR of MIL-118A. Reprinted with permission from Ref. [20]. Copyright 2009 American Chemical Society.45 Sc SSNMR. Scandium based materials own great potential in the application of catalyst, such as scandium(III) trifluoromethanesulfonate [Sc(OTf)3], which can be used to catalyze the carbon-carbon bond-forming reaction in aqueous media [24, 25]. Due to its good receptivity and moderate quadrupolar interactions as listed in Table 1, 45Sc has good NMR properties. Rossini et al. characterized the Sc(OTf)3 structure through the combination of 45Sc SSNMR and powder X-ray diffraction [26, 27]. For Sc(OTf)3 it is difficult to obtain single crystal causing dramatic difficulties in structural determination. They chose Sc(OAc)3 as a reference sample due to the well determined crystal structure. Meanwhile, the 45Sc SSNMR spectra of both Sc(OTf)3 and Sc(OAc)3 are quite similar, and the obtained EFG tensor parameters are almost identical. This suggests that the Sc coordination states are almost the same in both samples. On the basis of the 3D structure of Sc(OAc)3, the crystal structure of Sc(OTf)3 could be determined through the powder X-ray diffraction pattern.Besides catalysts, scandium based MOF can also be used in gas separation due to the porous structure. Miller et al. characterized the structure difference between scandium terephthalate, Sc2(BDC)3, and its derivatives, which is able to absorb CO2, through 45Sc SSNMR [28]. 45Sc SSNMR spectrum of Sc2(BDC)3 displays a characteristic second-order quadrupolar NMR line shape representing a single site for Sc. However, 45Sc SSNMR spectra of its derivatives, amino- and nitro-BDC, display featureless resonance line indicating statistical disorder of the orientation of the functionalized derivatives.69/71 Ga SSNMR. Gallium owns two NMR active nucleuses: 69Ga and 71Ga as listed in Table 1. As compared with 71Ga, 69Ga has larger CQ, which displays more broadened line shapes. Therefore, 71Ga is favored in SSNMR experiment. Compared with 27Al, 71Ga owns larger second-order quadrupolar splitting (wQ=3CQ/4I(2I-1)) due to its low spin number (I=3/2). This leads to broader NMR resonance lineshape. Meanwhile, the natural abundance of 71Ga is 40%, while that of 27Al is 100%. Therefore, either NMR sensitivity or resolution of 71Ga is lower than those of 27Al. In order to overcome these deficiencies, several software (new pulse sequence) and hardware (fast magic angle spinning, MAS) techniques are utilized. Summary of these techniques can be found in relevant reviews [29].Hajjar et al. investigated71Ga coordination environment in MIL-120 and MIL-124 through CT (central transition) MAS [30] NMR in conventional NMR probe [31]. In combination of the NMR simulation SIMPSON [32] and 2D slow-CTMAS results, EFG parameters of different crystallographic Ga sites are determined; there are two distinct Ga sites in MIL-120 and one in MIL-124 with remaining site undetectable due to fast molecular motion. This paper shows that classical NMR probe without ultrahigh magnetic field and ultrahigh spinning rate can also be used to detect 71Ga NMR signal and extract useful structural information. Ash and Grandinetti systematically investigated 69/71Ga SSNMR in different coordination state [33]. With the aid of ultrafast MAS spinning rate (30 kHz), the whole spectra of 69/71Ga NMR are able to be obtained. The isotropic chemical shift of 69/71Ga is found to be highly dependent on the coordination state: four coordinates lead to 50 ppm and six coordinates result in 225 ppm. Volkringer et al. studied gallium trimesate (MIL-96) through 71Ga SSNMR in high magnetic field [34]. 71Ga SSNMR spectrum in the magnetic field strengthB0 = 14.1 T shows a featureless single resonance line. However, under the higherB0 = 17.6 T, two distinct 71Ga signals can be observed. This enable us to extract the isotropic chemical shift and EFG parameters to identify different Ga sites in MOF system.25 Mg/ 47/49 Ti/ 91 Zr/ 67 Zn SSNMR. Despite above nucleuses, other frequently used metal ions in MOFs such as 25Mg, 47/49Ti, 91Zr, and 67Zn in MOFs experience more severe problems in NMR signal and sensitivity. As shown in Table 1, the natural abundance of these nucleuses is quite low (<12%). This requires more NMR experiment time to get a NMR spectrum with an acceptable signal-to-noise ratio. Moreover, the low gyromagnetic ratio γ (with corresponding Larmor frequency <50 MHz at 11.7 T) is beyond classical NMR probe sensitive range and large quadrupolar interactions (Q>150 MB) leads to broad quadrupolar lineshape. These unfavorable factors result in the low receptivity (<0.001). Therefore, both NMR sensitivity and resolution are extremely poor. To overcome these problems, ultrahigh magnetic field, ultrafast MAS spinning rate (≫10 kHz), and sensitivity enhancement techniques are necessary.Huang’s group used25Mg SSNMR to investigate the magnesium based MOF systems at high magnetic field (21.1 T) [35, 36]. The dehydration and rehydration process of CPO-27-Mg was in situ observed through 25Mg SSNMR [35]. The dehydration process leads to broadened 25Mg SSNMR lineshape indicating that the system becomes more disordered. And the dehydrated MOF presents featureless 25Mg NMR signal, which comes from the large CQ resulted from the distorted symmetry.In literature,47,49Ti SSNMR was normally obtained through sensitivity enhancement pulse sequences, such as fast amplitude-modulated pulse trains [37] or high magnetic field [16, 38, 39]. Due to almost the same Larmor frequency, 47Ti and 49Ti NMR signals always overlap with each other. Gervais et al. investigated the phase distribution of Titania nanoparticle through 47,49Ti SSNMR. Due to different isotropic chemical shift and EFG parameters for different Ti, 47,49Ti SSNMR can be used to get quantitative information of different Ti species with different local symmetry. Rossini et al. studied Titanocene Chlorides through the combination of 47,49Ti SSNMR and quantum chemical calculation [39]. In this case, 47Ti and 49Ti NMR spectra were observed separately at 21.1 T. The obtained NMR line shape together with EFG parameters and chemical shift (CS) tensors clarifies the Ti local structure and the symmetry.91Zr SSNMR was first reported in 1991 to investigate synthetic oxide materials [40]. 91Zr SSNMR line shape was found to be highly dependent on local symmetry. Zhu et al. investigated 91Zr SSNMR in two zirconium silicates AM-2 and AV-3 [41]. It was found that not only the local symmetry and distortion of the [ZrO6] but also the second and third spheres of atoms could influence NMR line shape. This shows that the quadrupolar interaction and CS tensor could also reflect surrounding environment.Sutrisno et al. studied Zn-contained MOFs (zeolitic imidazolate frameworks ZIF-8, ZIF-14, ZIF- 4, and ZIF-7) through67Zn SSNMR [42]. The 67Zn NMR line shape is determined not only by the local environment but also by the guest molecules. In combination with molecular dynamics, detailed information of the distribution and dynamics of guest molecules can be obtained. This paper, for the first time, shows the potential of 67Zn SSNMR in determining the detailed structure of Zinc based MOFs as a powerful complimentary characterization tool to X-ray diffraction.


## 2.2. Identity of Organic Linker

Despite the central metal ion, another important building block of MOFs is organic linker, or organic ligand. The organic linker is mostly constituted by carbon, proton, and oxygen.13C and 1H SSNMR are frequently used to identify the presence or absence of certain functional groups. As shown in Figure 4, 13C and 1H NMR spectra of MIL-53 in different phases display characteristic NMR line features [8]. 13C direct polarization (DP) MAS NMR spectrum could provide quantitative information of 13C of different functional groups as the integrated intensity is proportional to the number of 13C. As shown in Figure 4(a), MIL-53as (Al) displays relatively broad NMR resonance line suggesting the heterogeneity of the system. However, after thermal treatment at high temperature, a narrow and sharp 13C resonance line could be observed suggesting a highly ordered structure with high crystallinity. Further incorporation of guest molecules (i.e., water) broadens and shifts NMR resonance line indicating strong interaction between host (MIL-53) and guest (water) molecules. 13C cross polarization (CP) MAS NMR, most of the time, provides us with qualitative information about different 13C species due to the cross polarization efficiency highly relying on the interaction between observed 13C and neighboring 1H, which can be modulated by various factors, such as internuclear distance and molecular motion. As shown in Figure 4(b), when incorporated with guest molecule (water), the carbonyl carbon NMR signal almost disappears as a result of lower CP efficiency caused by the enhanced molecular motions.Figure 4
(a)13C DPMAS NMR spectrum, (b) 13C CPMAS NMR spectrum, and (c) 1H MAS NMR spectrum of MIL-53 in different phases. Reprinted with permission from Ref. [8]. Copyright 2004 John Wiley and Sons.
(a) (b) (c)1H SSNMR usually suffers from poor resolution resulting from the small chemical shift range and strong homonuclear dipolar coupling (~60 kHz). With great advances in both hardware (i.e., fast MAS > 30 kHz) and software (new pulse sequence), the NMR resolution of proton spectrum has been largely increased. Nowadays, 1H SSNMR has been widely used to study complicated systems, such as supramolecules [43–45] and biological areas [46–49]. As shown in Figure 4(c), up to three distinct proton signals can be obtained in MIL-53as (Al). Upon thermal treatment, the free carboxylic acid is completely removed, and further hydration treatment introduces the signal of water. Meanwhile, the hydroxyl group, Al-OH-Al, is directly observed throughout all three samples.Some MOFs contain paramagnetic metal center such as Cu3(BTC)2 (copper (II) benzene 1,3,5-tricarboxylate) [50]. The introduction of paramagnetic ions could largely influence the local magnetic field [51]. It will change the 1H and 13C chemical shift, broaden the NMR resonance line, and reduce the spin-lattice relaxation time [52]. Gul-E-Noor et al. found that the proton signal of water absorbed by Cu3(BTC)2 can shift downfield about 7.0 ppm [50]. For 13C SSNMR spectrum, signal of aromatic part shifts downfield of 98–92 ppm, while that of the carbonyl carbon shifts upfield of ~300 ppm. The possible origin of such difference is ascribed to closer interaction of copper and carboxylic groups.Oxygen is another frequently encountered atom in MOFs as organic carboxylate ligand is an important building block for many MOFs. Also, other functional groups such as hydroxyl (-OH-), oxygen anion (O2-) interacting with metal ions, and guest molecules (i.e., water) contain oxygen atom. However, the only NMR active isotope is 17O, whose natural abundance is 0.038%. 17O enrichment is necessary to do 17O SSNMR leading to high cost. As a result, little literature about 17O SSNMR of MOFs has been reported. He et al. reported an innovative method to prepare 17O enriched MOFs and characterize 17O of different sites through SSNMR [53]. 17O enriched MOFs are prepared through incorporating 17O atom from H2O17. The EFG parameters and chemical shift tensor information can be obtained through 17O SSNMR. Thus providing detailed structural information and identification of different oxygen contained species.


## 3. Investigation of Dynamics in MOF by SSNMR

Molecular dynamics is another key aspect for materials scientist to understand the correlation between microscopic and macroscopic properties. A thorough understanding of molecular dynamics requires characterization dynamics range of various techniques which can cover up to 14 orders of magnitude (10−12–102 s). SSNMR is one of the most frequently utilized techniques to characterize the molecule dynamics through relaxation or different anisotropic interactions (i.e., CSA and dipole-dipole interaction). Numerous reviews have been reported elsewhere [10–12, 54, 55]. Here only selected literature is reported to show how SSNMR can be used to characterize different functional groups in MOFs.


### 3.1. Phase Transition as Investigated by Proton Spin-Lattice Relaxation

Organic linker of MOFs always performs ultrafast dynamics (τc<10−10 s) at ambient temperature. Proton spin-lattice relaxation (T1) is able to probe such fast dynamics. Besara et al. investigate the phase transition and glassy behavior of [(CH3)2NH2] Zn(HCOO)3 [56]. This MOF takes an order-disorder transition at 156 K. The cationic [(CH3)2NH2]+ (DMA+) is used as a probe to track the phase transition behavior. As shown in Figure 5, the minima ofT1 detected at 156 K is assigned to the order-disorder transition, and the shoulder observed at 40 K is attributed to the transition of the glassy phase to ferroelectric (FE) phase. Below 40 K, the motion of the methyl group is frozen. The “memory effect” was observed at the range of 65–150 K range as shown byT1 difference resulted from different pathways.Figure 5
The protonT1 as a function of the inverse temperatureT−1. A, B, and C refer to glassy, ferroelectric (FE), and paraelectric (PE) states. And two characteristic temperatures are 40 K and 156 K. Reprinted with permission from Ref. [56]. Copyright 2011 PNAS.Morris et al. utilized proton spin-lattice relaxation method to probe dynamics of organic linkers in IRMOF-3 (Zn4O(BDC-NH2)3) [57]. Besides 1HT1, the spin-lattice relaxation in the rotating frame (T1ρ) under a fixed spin-locking field was also used to investigate dynamics of different groups. Different fromT1, which is sensitive to ultrafast dynamics (~MHz),T1ρ could detect dynamics ~10 kHz. Due to well resolved proton signal of amino and phenyl groups, molecular dynamics of each functional group can be determined. Their finding reveals that the amino group performs rotation with low activated energy and the central phenyl group (BDC) performs π flip motion with an activation energy of 21 kJ/mol.


### 3.2. Deuterium (2H) NMR Spectroscopy

Deuterium (2H) SSNMR is a versatile NMR technique to characterize the molecular dynamics with high resolution and sensitivity. Different from above metal ions, deuterium NMR is dominated by the first-order quadrupolar interaction. The quadrupolar echo pulse is used to obtain an undistorted anisotropic spectrum [58]. Due to its simple pulse sequence and data acquisition, deuterium NMR owns wide application, even in industrial areas [59]. Various 2H NMR techniques, including 1D lineshape analysis and 2D exchange spectroscopy, have been applied to characterize segmental dynamics of glassy polymers (10−6–102 s) [59, 60]. Therefore, deuterium SSNMR could be used to investigate the dynamics of different functional groups in MOFs.Bureekaew et al. investigated the aluminum MOFs with high conductivity through imidazole as a proton-carrier molecule [61]. Two similar Al MOFs, [Al(µ2-OH)(1,4-ndc)]n (1) and [Al(µ2-OH)(1,4-bdc)]n (2), show dramatic conductivity difference; MOF1 has higher proton conductivity than MOF2. They used 2H SSNMR to investigate the dynamics of imidazole in different porous systems. As shown in Figure 6, 2H SSNMR spectrum of imidazole-d4 in MOF2 at 20°C shows Pake-type doublet pattern suggesting anisotropic motion of adsorbed imidazole-d4, while that in MOF1 displays an extra Lorentzian-type peak located at the middle positon. This indicates isotropic motion of imidazole-d4. The comparison between MOFs1 and2 shows that the adsorbed imidazole owns different dynamics characteristic especially at higher temperature; imidazole performs fast and isotropic motion in MOF1 while it remains unchanged in MOF2. Such dynamics difference is associated with the proton conductivity difference, and the proton conducting pathway is also determined to be proton-hopping mechanism.Figure 6
2H NMR of [Al(µ2-OH)(1,4-ndc)]n (1,4-ndc: 1,4-naphthalenedicarboxylate) [66] and [Al(µ2-OH)(1,4-bdc)]n (1,4-bdc: 1,4-benzenedicarboxylate) loaded with imidazole-d4 molecules [61]. Reprinted with permission from Ref. [61]. Copyright 2009 Nature Publishing Group.


### 3.3. Chemical Shift Anisotropy

The NMR signal in solid state is orientation dependent [62]. For certain nucleus, such as 13C, this interaction is named chemical shift anisotropy (CSA) and can be used to probe molecular dynamics (10−3–102 s). For MOFs, as mentioned in the introduction, one of the most promising applications is the capture and storage of guest molecules, such as CO2. A molecular level understanding of the binding and dynamics of these guest molecules is necessary to refine current MOFs.Kong et al. for the first time uses SSNMR to characterize the binding and molecular dynamics of CO2 in Mg-MOF-74 (Mg2(dobdc) (H4dobdc = 2,5-dihydroxyterephthalic acid) [9]. CO2 is 13C enriched in order to obtain good signal-to-noise ratio. The temperature dependence of CSA patterns is acquired together with CSA simulation in order to obtain quantitative information about the molecular orientation of CO2. CO2 is found to perform uniaxial rotation close to Mg2+ site at a relatively fixed angle. Zhang et al. systematically investigated the molecular dynamics of guest molecule (CO2) in various MIL-53 through SSNMR [63]. The temperature dependent CSA pattern of 13C (13CO2) reveals the wobbling and hopping behavior of CO2 within MIL-53. The change of metal ion (from Al to Ga) could change CSA pattern suggesting the binding strength change, which is later confirmed by 1H-13C CP SSNMR.Despite porous MOF systems, the host-guest interaction also exists in some nonporous crystal at high temperature. The CO2 sorption process in these crystals is difficult to be observed due to its rapid motion. Bin et al. use synchrotron powder X-ray diffraction and SSNMR to investigate the restrained motion of CO2 in the small cages within the crystal [64]. The interaction between the host and guest molecules is characterized by 1H-13C HETCOR. The CO2 molecules are found to be located between the outer phenyl rings of host molecule H3BTB and the DMF molecules. Similar to above mentioned MOFs, the incorporation and molecular dynamics of CO2 are investigated through 13C CSA.


## 3.1. Phase Transition as Investigated by Proton Spin-Lattice Relaxation

Organic linker of MOFs always performs ultrafast dynamics (τc<10−10 s) at ambient temperature. Proton spin-lattice relaxation (T1) is able to probe such fast dynamics. Besara et al. investigate the phase transition and glassy behavior of [(CH3)2NH2] Zn(HCOO)3 [56]. This MOF takes an order-disorder transition at 156 K. The cationic [(CH3)2NH2]+ (DMA+) is used as a probe to track the phase transition behavior. As shown in Figure 5, the minima ofT1 detected at 156 K is assigned to the order-disorder transition, and the shoulder observed at 40 K is attributed to the transition of the glassy phase to ferroelectric (FE) phase. Below 40 K, the motion of the methyl group is frozen. The “memory effect” was observed at the range of 65–150 K range as shown byT1 difference resulted from different pathways.Figure 5
The protonT1 as a function of the inverse temperatureT−1. A, B, and C refer to glassy, ferroelectric (FE), and paraelectric (PE) states. And two characteristic temperatures are 40 K and 156 K. Reprinted with permission from Ref. [56]. Copyright 2011 PNAS.Morris et al. utilized proton spin-lattice relaxation method to probe dynamics of organic linkers in IRMOF-3 (Zn4O(BDC-NH2)3) [57]. Besides 1HT1, the spin-lattice relaxation in the rotating frame (T1ρ) under a fixed spin-locking field was also used to investigate dynamics of different groups. Different fromT1, which is sensitive to ultrafast dynamics (~MHz),T1ρ could detect dynamics ~10 kHz. Due to well resolved proton signal of amino and phenyl groups, molecular dynamics of each functional group can be determined. Their finding reveals that the amino group performs rotation with low activated energy and the central phenyl group (BDC) performs π flip motion with an activation energy of 21 kJ/mol.


## 3.2. Deuterium (2H) NMR Spectroscopy

Deuterium (2H) SSNMR is a versatile NMR technique to characterize the molecular dynamics with high resolution and sensitivity. Different from above metal ions, deuterium NMR is dominated by the first-order quadrupolar interaction. The quadrupolar echo pulse is used to obtain an undistorted anisotropic spectrum [58]. Due to its simple pulse sequence and data acquisition, deuterium NMR owns wide application, even in industrial areas [59]. Various 2H NMR techniques, including 1D lineshape analysis and 2D exchange spectroscopy, have been applied to characterize segmental dynamics of glassy polymers (10−6–102 s) [59, 60]. Therefore, deuterium SSNMR could be used to investigate the dynamics of different functional groups in MOFs.Bureekaew et al. investigated the aluminum MOFs with high conductivity through imidazole as a proton-carrier molecule [61]. Two similar Al MOFs, [Al(µ2-OH)(1,4-ndc)]n (1) and [Al(µ2-OH)(1,4-bdc)]n (2), show dramatic conductivity difference; MOF1 has higher proton conductivity than MOF2. They used 2H SSNMR to investigate the dynamics of imidazole in different porous systems. As shown in Figure 6, 2H SSNMR spectrum of imidazole-d4 in MOF2 at 20°C shows Pake-type doublet pattern suggesting anisotropic motion of adsorbed imidazole-d4, while that in MOF1 displays an extra Lorentzian-type peak located at the middle positon. This indicates isotropic motion of imidazole-d4. The comparison between MOFs1 and2 shows that the adsorbed imidazole owns different dynamics characteristic especially at higher temperature; imidazole performs fast and isotropic motion in MOF1 while it remains unchanged in MOF2. Such dynamics difference is associated with the proton conductivity difference, and the proton conducting pathway is also determined to be proton-hopping mechanism.Figure 6
2H NMR of [Al(µ2-OH)(1,4-ndc)]n (1,4-ndc: 1,4-naphthalenedicarboxylate) [66] and [Al(µ2-OH)(1,4-bdc)]n (1,4-bdc: 1,4-benzenedicarboxylate) loaded with imidazole-d4 molecules [61]. Reprinted with permission from Ref. [61]. Copyright 2009 Nature Publishing Group.


## 3.3. Chemical Shift Anisotropy

The NMR signal in solid state is orientation dependent [62]. For certain nucleus, such as 13C, this interaction is named chemical shift anisotropy (CSA) and can be used to probe molecular dynamics (10−3–102 s). For MOFs, as mentioned in the introduction, one of the most promising applications is the capture and storage of guest molecules, such as CO2. A molecular level understanding of the binding and dynamics of these guest molecules is necessary to refine current MOFs.Kong et al. for the first time uses SSNMR to characterize the binding and molecular dynamics of CO2 in Mg-MOF-74 (Mg2(dobdc) (H4dobdc = 2,5-dihydroxyterephthalic acid) [9]. CO2 is 13C enriched in order to obtain good signal-to-noise ratio. The temperature dependence of CSA patterns is acquired together with CSA simulation in order to obtain quantitative information about the molecular orientation of CO2. CO2 is found to perform uniaxial rotation close to Mg2+ site at a relatively fixed angle. Zhang et al. systematically investigated the molecular dynamics of guest molecule (CO2) in various MIL-53 through SSNMR [63]. The temperature dependent CSA pattern of 13C (13CO2) reveals the wobbling and hopping behavior of CO2 within MIL-53. The change of metal ion (from Al to Ga) could change CSA pattern suggesting the binding strength change, which is later confirmed by 1H-13C CP SSNMR.Despite porous MOF systems, the host-guest interaction also exists in some nonporous crystal at high temperature. The CO2 sorption process in these crystals is difficult to be observed due to its rapid motion. Bin et al. use synchrotron powder X-ray diffraction and SSNMR to investigate the restrained motion of CO2 in the small cages within the crystal [64]. The interaction between the host and guest molecules is characterized by 1H-13C HETCOR. The CO2 molecules are found to be located between the outer phenyl rings of host molecule H3BTB and the DMF molecules. Similar to above mentioned MOFs, the incorporation and molecular dynamics of CO2 are investigated through 13C CSA.


## 4. Conclusion

In this review, we briefly summarize different SSNMR work on MOFs. SSNMR is a powerful analytical technique to characterize metal ion center, organic linker, and host-guest interaction as a complimentary technique to X-ray diffraction. Meanwhile, SSNMR can characterize molecular dynamics over a broad range with site specific advantage at a molecular level. Such information can in turn help us understand the origin of some macroscopic performances of MOFs. This could rationalize the molecular design of MOFs. Summary of different MOFs characterized by different SSNMR techniques is shown in Table2. Admittedly, limited by article length, there is numerous pieces of SSNMR literature of MOFs which is not included in current review. With the increment of applications of MOFs and the great achievement of SSNMR techniques, it will be more convenient for us to obtain molecular information of different functional groups of MOFs, and SSNMR would become an indispensable analytical tool to characterize both structure and dynamics of MOFs.Table 2
Application of SSNMR in various MOFs with different techniques.
MOF SSNMR techniques Information Reference MIL-53 27Al, 1H, 13C SSNMR Structural determination of different phases;identification of different organic linker [8, 19] MIL-118 27Al 3QMAS NMR Quantitative determination of Al with different coordinate number [20] Sc2(BDC)3 45Sc SSNMR Orientation of Sc with different derivatives [28] MIL-120 & MIL-124 2D slow-CTMAS71Ga SSNMR Structural determination [31] MIL-96 71Ga SSNMR in high magnetic field Structural determination [34] CPO-27-Mg 25Mg SSNMR In situ observation of hydration and dehydration process [35, 36] ZIF-8, ZIF-14, ZIF- 4, ZIF-7 67Zn SSNMR Structural determination [42] MIL-125 47/49Ti SSNMR Determination of the presence and absence of guest molecule [65] Cu3(BTC)2 13C MAS NMR Influence of Cu(II) on13C NMR [50] C H 3 2 N H 2   Z n H C O O 3 1H T1 relaxation Phase transition [56] IRMOF-3 T 1ρ Molecular dynamics [57] A l µ 2 - O H 1,4- n d c n (1)  &   Alµ2-OH1,4-bdcn (2) 2H NMR Molecular dynamics [61] Mg-MOF-74 13C CSA and 13C-1H HETCOR Molecular dynamics and orientation of CO2 [9]



---

*Source: 6510253-2016-12-14.xml*