Article pubs.acs.org/cm
Mapping Out Chemically Similar, Crystallographically Nonequivalent Hydrogen Sites in Metal−Organic Frameworks by 1H Solid-State NMR Spectroscopy Jun Xu,† Victor V. Terskikh,‡ Yueying Chu,§ Anmin Zheng,*,§ and Yining Huang*,† †
Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada § State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, P.R. China ‡
S Supporting Information *
ABSTRACT: Metal−organic frameworks (MOFs) are important materials with many actual and potential applications. Crystal structure of many MOFs is determined by single-crystal X-ray diffraction. However, due to the inability of XRD to accurately locate hydrogen atoms, the local structures around framework hydrogen are usually poorly characterized even if the overall framework has been accurately determined. 1H solid-state NMR (SSNMR) spectroscopy should, in principle, be used as a complementary method to XRD for characterizing hydrogen local environments. However, the spectral resolution of 1H SSNMR is severely limited by the strong 1H−1H homonuclear dipolar coupling. In this work, we demonstrate that highresolution 1H MAS spectra of MOF-based material can be obtained by ultrafast sample spinning at high magnetic field in combination with isotopic dilution. In particular, we examined an important MOF, microporous α-Mg3(HCOO)6 and αMg3(HCOO)6 in the presence of several guest species. All six chemically very similar, but crystallographically, nonequivalent H sites of these MOFs were resolved in a chemical shift range as small as 0.8 ppm. Although the assignment of 1H peaks due to crystallographically nonequivalent hydrogens is difficult due to that they all have almost identical chemical environments, we are able to show that they can be assigned from 1H−1H proximity maps obtained from 2D 1H−1H double quantum (DQ) experiments in conjunction with theoretical calculations. 1H MAS spectra of framework hydrogen are very sensitive to the guest molecules present inside the pores and they provide insight into host−guest interaction and dynamics of guest molecule. The ability of achieving very high resolution for 1H MAS NMR in MOF-based materials and subsequent spectral assignment demonstrated in this work allows one to obtain new structural information complementary to that obtained from single-crystal XRD.
■
INTRODUCTION
interactions can strongly influence the adsorptive and other properties of MOFs. In principle, 1H solid-state NMR (SSNMR) spectroscopy should be used to directly probe the environment of framework hydrogens. However, unlike in solution-state where 1H NMR spectra of very high resolution can be acquired routinely, 1H NMR spectroscopy of rigid solids is very challenging due to the very strong proton−proton homonuclear dipolar interaction resulting from both high proton spin density and large gyromagnetic ratio (γH). Consequently, solid-state 1H NMR spectra are almost always dominated by this strong anisotropic interaction, leading to an extremely broad and featureless signal. In recent years, much progress has been made in achieving high resolution for 1H NMR in solids. Among others, spinning
Metal−organic frameworks (MOFs) are a class of inorganic− organic hybrid porous materials with three-dimensional frameworks. Because of their promising properties such as rich structural diversity, high thermal stability, and exceptionally large surface area, MOF-based materials have attracted tremendous attention to a broad range of applications.1 Understanding the relationship between the property of MOFs and their structures is essential to their applications. Although the structures of many MOFs have been determined from single crystal X-ray diffraction (XRD), it is extremely difficult, if not impossible, to locate framework protons by XRD. Characterizing the local environments around framework proton is of fundamental importance as the framework hydrogen atoms of organic linker and hydroxyl group are often involved in the host−guest interactions with adsorbed species via hydrogen bonding or van der Waals forces. These © XXXX American Chemical Society
Received: January 28, 2015 Revised: April 9, 2015
A
DOI: 10.1021/acs.chemmater.5b00360 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
CRYSFIRE powder indexing system.32 The guest content was measured by thermogravimetric analysis (TGA) (Supporting Information Figure S2). The samples were heated under a N2 atmosphere on a Mettler Toledo TGA/DTA851e instrument from 25 to 500 °C at a heating rate of 10 °C/min. NMR Characterizations. 1H and 13C SSNMR experiments were performed at 21.1 T on a Bruker Avance II spectrometer at the National Ultrahigh-Field NMR Facility for Solids in Ottawa, Canada. One-pulse 1H MAS spectra were acquired using a 1.3 mm H/X MAS Bruker probe with a spinning rate of 62.5 kHz, and a 4 mm HCN MAS Bruker probe with a spinning rate of 18 kHz. A 1H pulse length of 1.0 μs, corresponding to a 45° pulse, was used for MAS experiments performed on the 1.3 mm probe. A 90° pulse (2.5 μs) of 1H was used for MAS experiments performed on the above-mentioned 4 mm probe. The static 1H NMR spectra were collected using a spin−echo sequence. The interpulse delay τ was 60 μs. Rotor-synchronized 2D 1 H−1H DQ BABA experiments33 were performed using a 4 mm probe at 18 kHz MAS. A total of 128 increments (or 128 t1 experiments) in the indirect DQ dimension, each with 16 scans, was collected. The BABA scheme allows the recoupling of the dipolar coupling, which is averaged by MAS. Excitation/reconversion over one rotor cycle (55.6 μs), two rotor cycles (111.1 μs), and four rotor cycles (166.7 μs) were used. All 1H pulse lengths were measured on adamantane and the chemical shift of 1H was also referenced to adamantane at 1.74 ppm relative to tetramethylsilane (TMS). 1H → 13C cross-polarization (CP) MAS experiments were carried out using the 4 mm HCN MAS Bruker probe, with a spinning rate of 18 kHz and a contact time of 2 ms. Because deuterated guest molecules were used, the guest species do not yield 13C CP signals. Thus, all the peaks observed in the CP spectra are from the framework only. All 1H → 13C CPMAS measurements were carried out with high-power 1H decoupling using the two-pulse-phase-modulation (TPPM) scheme.34 The phase increment of TPPM decoupling was 15° and the TPPM pulse length was 4.58 μs. The chemical shift of 13C was referenced to the −COOH signal of glycine at 176.5 ppm. 2D 1H−13C frequency switched Lee− Goldberg heteronuclear correlation (FSLG-HETCOR) experiments35 were performed at 18 kHz MAS with a very short contact time of 35 μs to avoid unwanted long-range correlations. 64 points with six Lee− Goldberg cycles as the increment of indirect (1H) dimension and 256 scans for each point were collected. To understand the dynamics of guest species, static 2H SSNMR spectra of d6-benzene- and d5pyridine-loaded α-Mg3(HCOO)6 were measured on a Varian Infinity Plus 400 WB spectrometer at 61.3 MHz at a magnetic field of 9.4 T using a horizontal 5 mm static probe and quadrupole echo sequence.36 The 90° pulse of 2H was 3.6 μs and the interpulse delay was 30 μs. Other details of SSNMR experiments, including the number of scans and pulse delays, are shown in Supporting Information Table S2. Theoretical Calculations. Gauge-including projector augmented wave (GIPAW) quantum chemical calculations were conducted using the CASTEP code 37 within Accelrys Materials Studio. SCF convergence criteria and k-point sampling were set using the “fine” criteria option implemented within CASTEP. Structural optimization of activated, DMF- and benzene-loaded samples was performed prior to the calculation of NMR parameters. Because the structures of αMg3(HCOO)6 samples are quite complicated, that is, with a large number of atoms in the unit cell as well as a large number of crystallographically nonequivalent sites, only the position of framework hydrogen atoms was allowed to move. Two approaches were used in H-optimization: The first one (the results are hereafter referred to as structure I) involved no dispersion correlations (using CASTEP version 4.4) and a very low plane-wave cutoff energy of 450 eV, and the unit cell parameters were taken from single crystal structures at 100 K,15 whereas the second one (the results are hereafter referred to as structure II) involved dispersion correlations (Grimme custom method for DFT-D, using CASTEP version 7.0) and a markedly higher cutoff energy of 550 eV, and the unit cell parameters were adjusted according to room temperature (293 K) powder XRD patterns to compensate the effect of thermal expansion. For both approaches, atomic coordinates were taken from single crystal structures measured at 100 K.15 All geometry optimizations used
sample at magic angle with ultrafast rates (60 kHz or higher) can dramatically improve the resolution of solid-state 1H NMR spectra due to the inverse rate dependence of the 1H line width.2−9 Performing 1H SSNMR experiments at very high magnetic fields provides additional benefits to spectral resolution since the 1H line width (in parts per million) is also inversely proportional to the magnetic field strength.9 The resolution can be further enhanced by diluting 1H spin density via deuteration.10−14 In this work, we demonstrate that by a combination of isotopic dilution, ultrafast magic-angle spinning (MAS), and performing 1H NMR experiments at a very high magnetic field, very high resolution of 1H SSNMR spectra can be achieved for MOF-based materials. The material examined is microporous α-Mg3(HCOO)6, which is an important commercialized MOF (trade name: Basosive M050).15−17 One reason for choosing this MOF is that good single-crystal structures are available for activated phase and several guest-loaded complexes, against which the reliability and validity of NMR approach can be checked. In addition, this MOF represents a very challenging situation for 1H SSNMR study because it has six crystallographically nonequivalent framework H sites with very similar local environments, making the spectral assignment extremely difficult. Several microporous α-Mg3(HCOO)6 samples were studied: activated phase (empty framework) and three guestloaded phases (hereafter referred to as activated, DMF-, benzene-, and pyridine-loaded phases, respectively). We show that using the above-mentioned resolution enhancement strategies, 1H SSNMR spectra of framework hydrogen with very high resolution can be obtained and they are extremely sensitive to the guest species present inside the channels. The very high spectral resolution achieved allows us to unambiguously determine the number of crystallographically nonequivalent hydrogens in the framework and extract valuable information on host−guest interaction. It should be pointed out that several studies of MOFs involving using 1H SSNMR have been reported in the literature.18−31 However, they focused on identifying chemically nonequivalent framework proton species in a wide chemical shift range (several parts per million). As shown below, the present work pushes the highresolution 1H SSNMR spectroscopy to the limit by resolving six crystallographically nonequivalent hydrogens with the same chemical origin in a very small chemical shift range less than 0.8 ppm.
■
EXPERIMENTAL SECTION
Sample Preparation. As-made fully protonated sample, αMg3(HCOO)6, was prepared following the method described by Rood and co-workers.15 As-made deuterium-diluted sample, αMg3(H/DCOO)6, (∼20% H, measured by mass spectrometry analysis) was synthesized using a mixture of HCOOH/DCOOH (DCOOH: CDN Isotopes, 99% D) with a molar ratio of 1:5 as the starting material. The activated phase was obtained by removing solvent from the as-made phase under dynamic vacuum at 150 °C for 1 d. To prepare the three guest-loaded phases (DMF, benzene, and pyridine), ∼ 0.5 g of activated sample was soaked in 1 mL of fully deuterated dry solvents (Cambridge Isotopes, 99.5% D) for 1 d. The excess solvent was evaporated under a N2 atmosphere. All samples were stored in sealed vials. The identity of the samples was examined by powder X-ray diffraction (PXRD). The PXRD patterns (Figure S1, Supporting Information) were recorded on a Rigaku diffractometer equipped with a graphite monochromator using Co Kα radiation (λ = 1.7902 Å). Diffraction data were collected from 5° to 45° in 2θ at a step size of 0.02°. The unit cell parameters at 293 K (Supporting Information Table S1) were refined from PXRD patterns using the B
DOI: 10.1021/acs.chemmater.5b00360 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 1. Structure of the DMF-loaded sample. Hydrogen atoms of the encapsulated DMF are omitted for clarity. ultrasoft pseudopotential. A MP grid size of 1 × 1 × 1 and a number of k-point of 1 were employed. The cif files of optimized structure are included in Supporting Information. The NMR module was employed to calculate NMR parameters based on the above-mentioned optimized structures. Three methods were used: The first one (the results are hereafter referred to as 450_111 in Supporting Information) utilized structure I, involving no dispersion correlations (using CASTEP version 4.4), a low cutoff energy of 450 eV, a MP grid size of 1 × 1 × 1 and a number of k-point of 1. The second one (the results are hereafter referred to as 550_111) utilized structure II, involving dispersion correlations (Grimme custom method for DFT-D, using CASTEP version 7.0), a moderate cutoff energy of 550 eV, a MP grid size of 1 × 1 × 1 and a number of k-point of 1. The third one (the results are hereafter referred to as 600_222) utilized structure II, involving dispersion correlations (Grimme custom method for DFT-D, using CASTEP version 7.0), a high cutoff energy of 600 eV, a MP grid size of 2 × 2 × 2 and a number of k-point of 2. All NMR parameter calculations used on-the-fly pseudopotentials. CASTEP calculation is a widely accepted approach for assigning 1H and 13C NMR resonances. However, in this particular case, CASTEP calculation is actually not a preferred method for spectral assignment because the local environments of framework proton and carbon are almost identical. We preformed calculations using different parameters and methods described above to show that the assignment based on calculated chemical shielding indeed varies with the parameters used. 1 H and 13C isotropic chemical shielding constants (Supporting Information Table S3 and S4) were produced automatically by the CASTEP program, which can be converted to isotropic chemical shifts using the relationship: δiso = σiso(Reference) − σiso(CASTEP) (in parts per million), where σiso(Reference) is the isotropic chemical shielding constant of primary reference. However, it is difficult to obtain accurate σiso(Reference) as the reference shielding values reported in the literature are inconsistent. In this paper, the assignments of 1H and 13 C peaks (Supporting Information Figure S3 and S4) based on CASTEP calculations were made according to the order of shielding constant.
Mg1, forming a 1D zigzag channel along the b direction with a window dimension of 4.5 × 5.5 Å. The chemical environment of six H (and C) sites is very similar: all formate anions adopt the same bonding mode where one oxygen bridges two Mg centers (μ2-oxygen) and the other one connects to a single Mg center (μ1-oxygen). The static (nonspinning) 1H spectrum of fully protonated (100% H) activated sample was first acquired at 21.1 T (Figure 2, top). It consists of a very broad and featureless profile with a
■
RESULTS AND DISCUSSION 1D Ultrahigh-Resolution 1H MAS Spectra: Resolving Six Crystallographically Nonequivalent Proton Sites in the Lattice. Single-crystal XRD data of activated, DMF- and benzene-loaded phase at 100 K indicate that their structures belong to the same space group (P21/n),15 containing four crystallographically nonequivalent Mg sites and six nonequivalent H and C sites. The multiplicity of six H (and C) sites is equal. Our powder XRD data (Supporting Information Figure S1 and Table S1) reveal that the room temperature (293 K) structures also belong to P21/n and the unit cell volumes at 293 K are slightly larger due to thermal expansion. As shown in Figure 1, the framework is constructed by linking onedimensional chains of edge-shared MgO6 octahedra of Mg1 and Mg3 with corner-shared octahedra of Mg2 and Mg4 via
Figure 2. Illustration of the resolution enhancement for the activated sample by MAS combined with isotopic dilution.
spectral breadth of ∼30 kHz. The shortest distance between two framework protons, measured from the reported crystal structure,15 is around 3 Å in the activated sample, resulting in strong dipolar coupling between neighboring protons. Moreover, each framework proton has several neighboring protons with similar distances, resulting in a dense network of dipolar coupling. The first resolution-enhancement approach used is C
DOI: 10.1021/acs.chemmater.5b00360 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 3. (Left) Experimental and deconvoluted 62.5 kHz MAS 1H spectra of four 20% H samples. The protons labeled with red exhibit significant guest-induced shifts. (Right) Experimental and deconvoluted 18 kHz 1H → 13C CPMAS spectra of four 20% H samples.
ultrafast MAS, because the line broadening due to 1H−1H homonuclear dipolar coupling can be reduced by fast MAS and the residual 1H line width is approximately inversely proportional to the spinning rate.2−6 Two spinning rates were employed (Figure 2, bottom): 18 and 62.5 kHz. Spinning sample at 18 kHz does narrow the proton signal significantly, but the resolution of the spectrum is still low. Spectral resolution was improved markedly when the sample was spun at 62.5 kHz. The resulting spectrum now exhibits four overlapping signals. To further enhance the resolution, isotopic dilution was then combined with ultrafast MAS. Partial substitution of 1H with 2H weakens proton−proton dipolar coupling network owing to a much smaller gyromagnetic ratio of 2H (∼15% of that for 1H).38 After substituting ∼80% of framework 1H by 2H (this sample is hereafter referred to as the 20% H sample), the corresponding MAS spectra (Figure 2, bottom) spun at 18 and 62.5 kHz were acquired. The resolution of the 18 kHz MAS spectrum of the 20% H sample is not very high, but the 62.5 kHz spectrum now clearly exhibits five sharp 1H lines within a narrow chemical shift range of ∼0.8 ppm. Among the five peaks, four have similar intensities, and the most intense peak (at ∼7.8 ppm) has a pronounced shoulder, which can be unambiguously deconvoluted into two peaks. In the deconvoluted spectrum of activated phase (the 20% H sample), six 1H signals were clearly resolved (Figure 3, left). The 1H line width of the observed peaks is in the range of 35−53 Hz (0.04−0.06 ppm), which is very narrow for a rigid
solid. Observing six 1H peaks with equal intensities is consistent with the proposed single-crystal structure. The 20% H samples loaded with d7-DMF and d6-benzene were also studied. The 1H MAS spectrum of these two samples spun at 62.5 kHz is shown in Figure 3 (left). Four well-resolved peaks plus a shoulder are clearly visible in the spectrum of the DMF-loaded sample, whereas there are five peaks for the benzene-loaded sample. For both DMF- and benzene-loaded samples, deconvoluted spectra indicate six nonequivalent framework H sites with approximately equal intensity, agreeing well with the proposed crystal structures. The 1H chemical shift of these framework protons is all in the normal range of formate-containing materials (∼8 ppm).39 It is worth noting that although the spectra of activated, DMF- and benzeneloaded sample all show six nonequivalent framework protons, they look distinctly different, showing the very high sensitivity of 1H MAS NMR to the local H environment. Because adsorption of guest species does not induce any change in either framework topology or crystal symmetry,15 this remarkable difference in spectral appearance must result from the presence of different guest species. To properly interpret three 1H MAS NMR spectra of MOF framework, we need to assign the spectra first. It turns out that relating the six observed 1 H peaks to the six nonequivalent H sites is not straightforward. Unlike the protons in different functional groups that have characteristic chemical shift ranges, for the MOF samples examined here, the six nonequivalent framework protons are chemically very similar. D
DOI: 10.1021/acs.chemmater.5b00360 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 4. 2D 1H−1H DQ BABA spectra of the 20% H activated sample as a function of excitation/reconversion time. Diagonals (black dash lines) and red dashed lines are drawn to illustrate the autocorrelation peaks, whereas the horizontal red solid lines indicate the cross peaks. DQ peaks are labeled exactly at the calculated positions along the 1H double quantum dimension. Only new correlations are shown at longer excitation times for clarity. The neighboring protons around H2 are shown at top right.
2D 1 H− 1H Double-Quantum MAS Experiments: Assigning Six 1H Resonances. In recent years, firstprinciples calculation of NMR chemical shielding using GIPAW method implemented in the CASTEP code has proven to be a reliable method for spectral assignment, provided that an accurate crystal structure is known.40−43 Unfortunately, in the present case despite the fact that the single crystal data of these MOFs are of high quality, the hydrogen atoms of the activated material as well as the DMFand benzene-loaded samples were not located by single-crystal XRD. Instead, they were placed in the presumed positions with an equal H−C distance of 0.930 Å in all three phases.15 It is worth noting that this assumed H−C distance is significantly shorter than the normal H−C distances (∼1.10 Å) reported in other magnesium formates.44,45 For these reasons, the proton positions were optimized prior to the calculation of NMR parameters. The H−C distances in geometrically optimized structures range from 1.104 to 1.111 Å (Supporting Information Table S5). Hereafter, all the calculations and geometry analyses were performed on the H-optimized structures. The 1H chemical shielding constants calculated using the optimized structures are presented in Supporting Information Table S3. Because the local environment of
framework protons is very similar, it is not surprising that a small error in the H−C distance (or the use of different computation methods) can lead to a significant change in calculated 1H chemical shielding constants, which in turn alters the spectral assignment (Supporting Information Figure S3). Therefore, to unambiguously assign the 1H MAS spectra, a more robust method for assignment needs to be used. Although the strong 1H−1H homonuclear dipolar coupling is the major obstacle for achieving high resolution, it does provide the basis for several SSNMR techniques that can be used to determine the 1H−1H proximity because the dipolar coupling strength between two protons j and k, djk (in hertz), is inversely proportional to third power of internuclear distance rjk5 djk = −
μ0 γ 2ℏ 8π 2r jk3
(1)
where μ0 denotes the vacuum permeability, γ is the gyromagnetic ratio, and ℏ is the reduced Planck constant. Furthermore, it has been demonstrated that when a given proton site j has several neighboring protons with similar distances (i.e., in the presence of dense dipolar coupling network), the total “effective dipolar coupling” experienced by E
DOI: 10.1021/acs.chemmater.5b00360 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
data indicate the framework hydrogens in α-Mg3(HCOO)6, are rigid at 293 K.51 2D 1H−1H DQ BABA spectra of the activated sample (20% H), using one, two, and four rotor cycle excitation and reconversion BABA sequence (corresponding to 55.6, 111.1, and 166.7 μs), are illustrated in Figure 4. The horizontal red lines connecting the cross peaks between unlike spins are placed exactly at the positions calculated using the SQ frequencies. We set three DQ spectra to have the same contour levels to monitor the growth of DQ intensity at different excitation/reconversion time. It is evident that the intensity of all DQ peaks increases with increasing excitation/ reconversion time. However, some DQ peaks (e.g., the cross peaks at 16.08 ppm along the DQ dimension) were not observed at a shorter excitation/reconversion time. It has been demonstrated in the literature52 that the relative intensity of DQ peaks is a reliable indicator of the relative strength of dipolar coupling, that is, the intensity ratio of DQ peaks is approximately the ratio of the squares of the corresponding dipolar coupling constants (unless the dipolar coupling is very strong, i.e., djk > ∼ 7800 Hz). Because the effective dipolar coupling constants in three MOF samples (Supporting Information Table S6) are all significantly smaller than 7800 Hz, the DQ peaks that are not observed at shorter excitation/ reconversion times must correspond to the protons with weaker dipolar coupling compared to the DQ peaks that are always present. We first focused on autocorrelation peaks corresponding to the like spins because there are only 6 possible correlations between two like protons, whereas there are 15 correlations between two unlike protons. At 55.6 μs, only the peak at 8.14 ppm on the SQ dimension can be seen along the diagonal, which can be unambiguously assigned to H2 because the H2− H2 has the strongest dipolar coupling (5704 Hz, Supporting Information Table S6). At 166.7 μs, two additional autocorrelation peaks corresponding to the resonances at 7.94 and 7.84 ppm along the SQ dimension become very obvious. The two autocorrelation peaks must originate from H3 and H5 due to their moderate dipolar coupling strengths (1898 Hz for H3 and 880 Hz for H5, respectively). To further distinguish H3 from H5, we examined the cross peaks involving H2. As Figure 4 shows, the cross peaks involving H2 and the SQ peaks at 7.55, 7.84, and 8.07 ppm are already present at 55.6 μs, whereas the cross peaks involving H2 and the SQ peak at 7.94 ppm can only be observed at 111.1 μs or longer. According to the crystal structure, there are four neighboring nonequivalent protons (H3, H4, H5, and H6) in the spatial proximities of H2 (Figure 4), among which the dipolar coupling strength between H2 and H3 (1342 Hz) is significantly weaker than those between H2 and the other protons. Therefore, we assigned the SQ peak at 7.94 ppm to H3 and the SQ peak at 7.84 ppm to H5. Similarly, the cross peaks involving H3 and the SQ peak at 8.07 ppm, which were not observed at 55.6 μs, correspond to the weaker dipolar coupling of H3−H4 (1591 Hz) compared to those of H3−H1 (3101 Hz), H3−H5 (2333 Hz), and H3−H6 (2587 Hz). In addition, the cross peaks involving H4 and the SQ peak at 7.55 ppm, which are unobserved at 55.6 μs, is owing to H4− H6. As a result, the SQ peak at 7.81 ppm can only be H1. The assignment of H2 and H4 is worth commenting as there is a potential that H2 and H4 could be assigned the other way around. Although the autocorrelation peak at 16.28 ppm overlaps with H2−H4 cross peaks, it is due to H2−H2, and not H4−H4. The effective dipolar coupling constants are 5843 and
proton site j can be expressed quantitatively using the rootsum-square dipolar coupling4,6,46 drss, j =
Σk ≠ jdjk2
(2)
As the literature demonstrates,6,46 the effective dipolar coupling constant can be viewed as a site-specific version of the second moment which has been widely used to measure the dipolar coupling strength in static samples where individual proton sites are not resolved. One of the dipolar coupling-based SSNMR techniques that is capable of determining 1 H− 1 H proximity is a rotorsynchronized two-dimensional (2D) 1H−1H double-quantum (DQ) MAS experiment using the back-to-back (BABA) recoupling sequence.30,31,47 Double-quantum signals can be yielded if two protons are close enough to be dipolar coupled. In a 2D 1H−1H DQ BABA spectrum such as those shown in Figure 4, the frequencies (or the chemical shifts) in the DQ dimension are the sum of the 1H single-quantum (SQ) frequencies from two protons in close proximity coupled via dipolar interaction.48−50 Double-quantum coherences (DQCs) between equivalent protons (i.e., like spins) appear on the diagonal as autocorrelation peaks at twice the frequency of the SQ peaks. The dipolar-coupled protons with different chemical shifts (i.e., unlike spins) appear as a pair of cross peaks on either side of the diagonal. A recent literature49 shows that the DQ build-up curve corresponding to motionally averaged weak dipolar coupling (∼250 Hz) can be measured. In this work, the assignment of 2D 1H−1H DQ BABA spectra was made based on the “effective dipolar coupling”. For α-Mg3(HCOO)6, there are six crystallographically nonequivalent H sites and the multiplicity is 4 for all the sites in one unit cell. The “effective dipolar coupling” between a given proton site j and all the surrounding crystallographically equivalent site k in the lattice was calculated as ∞
drss, jk =
∞
∑ d j2ki =
∑ d j2k
i=1
i=1
1
i 1
(3)
where dj1ki (i = 1, 2, 3, ...) denotes the dipolar coupling constant between a given proton j1 and crystallograhpically equivalent protons k1, k2, k3, and so forth, respectively, and vice versa. Only the proton pairs that are within a distance of 10 Å were included in the calculation and the drss,jk values are listed in Supporting Information Table S6. The details of calculating effective dipolar coupling constants are illustrated using H2− H2 and H2−H3 as examples and further discussion about the effects of cutoff distances (e.g., 20 and 40 Å) can be found in Supporting Information. It is worth noting that the effective dipolar coupling constants calculated using the aforementioned structure I are very similar to those calculated using structure II (the details of structures I and II are discussed in Experimental Section), although markedly higher cutoff energy, dispersion correlations, as well as room temperature unit cell parameters are employed in the latter case. This result indicates even if the H-optimized structures are not perfect, the assignments based on effective dipolar coupling are still valid. Hereafter, all the effective dipolar coupling constants used in discussion are calculated based on the structure II. Another issue is that the organic linker of many MOFs can possess significant thermal motions at room temperature, which may partially average the dipolar interaction. However, our previous 2H wide-line NMR F
DOI: 10.1021/acs.chemmater.5b00360 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
reasonable period of time. The spectrum shown in Supporting Information Figure S7 was acquired using 96 t1 points with each slice being acquired with 16 transients using a pulse delay of 60 s, resulting in a total of 25.6 h acquisition time. Using 18 kHz MAS for 2D 1H−1H DQ experiments appears to be a good compromise. Information on Host−Guest Interaction Extracted from 1H MAS Spectra. The spectral assignments allow us to understand the differences in the observed 1H MAS spectrum of three samples and to gain insight into the host− guest interaction. We first compare the 1H MAS spectrum of activated (unloaded) α-Mg3(HCOO)6 to that of the αMg3(HCOO)6 loaded with d7-DMF as the guest molecule. As mentioned earlier, upon adsorption of DMF, not only the topology of the α-Mg3(HCOO)6 framework remains the same, the crystal symmetry is also retained.15 The NMR spectra are consistent with the XRD results because the 1H MAS spectra (Figure 3) show that the DMF-loaded phase also has six crystallographically nonequivalent framework protons. However, the occlusion of DMF into the framework indeed induces the marked changes in 1H resonance frequency of these nonequivalent hydrogens (Figure 3 and Supporting Information Table S3). The observed frequency change must result from the interaction between guest molecule and framework protons. The largest changes occur to H5 and H6. Upon adsorption, H5 shifted toward the deshielded direction by 0.59 ppm from 7.84 ppm in the empty MOF to 8.43 ppm and became the most deshielded proton. The large high-frequency shift of H5 upon adsorption of DMF is likely due to the formation of a weak C−H···O hydrogen bond between the framework C−H fragment and the oxygen in DMF molecule because it is well established that the formation of such hydrogen bonding leads to the high-frequency shift of proton.8,54−56 A closer inspection of the local environment around DMF molecule (Figure 5) reveals that three framework
866 Hz for H2−H2 and H4−H4, respectively. If the autocorrelation peak at 16.28 ppm were due to H4−H4, a strong autocorrelation peak would be seen at 16.14 ppm due to much stronger H2−H2 dipolar coupling. As evident from Figure 4, no autocorrelation peak appears at 16.14 ppm on the diagonal of the spectrum with the shortest excitation time. Thus, the autocorrelation peak at 16.28 ppm originates from H2−H2. The final assignment of six H sites is given in Figure 3 and Supporting Information Table S3. We also matched the effective dipolar coupling strengths with the presence of DQ peaks at different excitation/reconversion time (Table 1), ensuring that the DQ peaks that can only be observed at longer time always relate to protons with weaker dipolar coupling. Table 1. Observed DQCs, δDQa, and Effective Dipolar Coupling Constants of the Activated Sample τexc (μs) 55.6
166.7
a
DQC H2−H2 H4−H5 H2−H4 H1−H6 H2−H6 H1−H3 H1−H4 H2−H5 H3−H6 H5−H6 H1−H5 H3−H5 H4−H6 H3−H3 H3−H4 H1−H2 H2−H3 H5−H5 H4−H4 (Not observed) H1−H1 (Not observed) H6−H6 (Not observed)
δDQ (ppm)
effective dipolar coupling constant (Hz)
16.28 15.91 16.21 15.36 15.69 15.75 15.88 15.98 15.49 15.39 15.65 15.78 15.62 15.88 16.01 15.95 16.08 15.68 16.14
5704 3992 3498 3279 3128 3101 2926 2661 2587 2478 2360 2333 2005 1898 1591 1360 1342 880 858
15.62
757
15.10
755
The sum of experimentally observed 1H SQ frequencies.
In a similar fashion, the 1H MAS spectrum of the DMF- and benzene-loaded samples was also assigned via examining 1 H−1H proximities (their 1H−1H DQ BABA spectra are shown in Supporting Information Figure S5 and S6 and the analyses are shown in Supporting Information Table S7 and S8). The final assignment of the six crystallographically nonequivalent H sites in each sample is given in Figure 3 and Supporting Information Table S3. It is worth mentioning that we have also performed 2D 1 H−1H DQ experiment at 62.5 kHz MAS on 20% H activated sample in attempt to see if higher MAS rate can further improve the 2D DQ MAS resolution. The spectrum is shown in Figure S7 in Supporting Information. The spectral resolution along the SQ (F2) dimension indeed improves significantly due to the increased spinning rate, but the resolution along the DQ (F1) dimension is a problem. This is because the 1H T1 under ultrafast spinning rates such as 62.5 kHz becomes very long53 and, therefore, only a limited number of t1 experiments can be performed in order to carry out the overall 2D experiment in a
Figure 5. Local environment around DMF in the DMF-loaded sample.
protons (H2, H5, and H6) are in close proximity (
