Victor V. Terskikh

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal-organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by (67)Zn solid-state NMR (SSNMR) spectroscopy. This is because (67)Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a (67)Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that (67)Zn magic-angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non-equivalent Zn sites. The (67)Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF-5 desolvation, we show that with the aid of computational modeling, (67)Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF-8 as an example, we further demonstrate that (67)Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of (67)Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes (67)Zn SSNMR spectroscopy as a new tool complementary to X-ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.

Application of Solid-State 209Bi NMR to the Structural Characterization of Bismuth-Containing Materials

Herein, we report the first detailed study of (209)Bi solid-state NMR (SSNMR) spectroscopy of extremely broad central transition powder patterns. (209)Bi ultrawideline SSNMR spectra of several bismuth-containing materials (bismuth oxyhalides, bismuth nitrate pentahydrate, nonaaquabismuth triflate, and bismuth acetate) were acquired at field strengths of 9.4 and 21.1 T using frequency-stepped techniques. The (209)Bi SSNMR experiments at 9.4 T yield powder patterns with breadths ranging from 0.9 to 14.6 MHz, from which quadrupolar coupling constants, C(Q)((209)Bi), between 78 and 256 MHz, were extracted via analytical simulations. The breadths of the quadrupolar-dominated spectra and overall experimental times are greatly reduced for experiments conducted at 21.1 T, which yield high signal-to-noise spectra in which the smaller effects of bismuth chemical shift anisotropy can be clearly observed. The (209)Bi electric field gradient (EFG) and chemical shift (CS) tensor parameters extracted from these spectra are correlated to the molecular structures at the bismuth sites, via first principles calculations of (209)Bi EFG and CS tensors performed using CASTEP for periodic solids and Gaussian 03 for molecular clusters. The rapidity with which (209)Bi SSNMR spectra can be acquired at ultrahigh fields, the sensitivity of the (209)Bi NMR parameters to the bismuth environment, and the predictive power of theoretically calculated NMR interaction tensors suggest that (209)Bi SSNMR may be useful for the characterization of a variety of Bi-containing materials and compounds.

Wobbling and Hopping: Studying Dynamics of CO2 Adsorbed in Metal–Organic Frameworks via 17O Solid-State NMR

Knowledge of adsorbed gas dynamics within microporous solids is crucial for the design of more efficient gas capture materials. We demonstrate that (17)O solid-state NMR (SSNMR) experiments allow one to obtain accurate information on CO2 dynamics within metal-organic frameworks (MOFs), using CPO-27-M (M = Mg, Zn) as examples. Variable-temperature (VT) (17)O SSNMR spectra acquired from 150 to 403 K yield key parameters defining the CO2 motions. VT (17)O SSNMR spectra of CPO-27-Zn indicate relatively weaker metal-oxygen binding and increased CO2 dynamics. (17)O SSNMR is a sensitive probe of CO2 dynamics due to the presence of both the quadrupolar and chemical shielding interactions, and holds potential for the investigation of motions within a variety of microporous materials.

Solid-State 17O NMR of Pharmaceutical Compounds: Salicylic Acid and Aspirin

We report solid-state NMR characterization of the (17)O quadrupole coupling (QC) and chemical shift (CS) tensors in five site-specifically (17)O-labeled samples of salicylic acid and o-acetylsalicylic acid (Aspirin). High-quality (17)O NMR spectra were obtained for these important pharmaceutical compounds under both static and magic angle spinning (MAS) conditions at two magnetic fields, 14.0 and 21.1 T. A total of 14 (17)O QC and CS tensors were experimentally determined for the seven oxygen sites in salicylic acid and Aspirin. Although both salicylic acid and Aspirin form hydrogen bonded cyclic dimers in the solid state, we found that the potential curves for the concerted double proton transfer in these two compounds are significantly different. In particular, while the double-well potential curve in Aspirin is nearly symmetrical, it is highly asymmetrical in salicylic acid. This difference results in quite different temperature dependencies in (17)O MAS spectra of the two compounds. A careful analysis of variable-temperature (17)O MAS NMR spectra of Aspirin allowed us to obtain the energy asymmetry (ΔE) of the double-well potential, ΔE = 3.0 ± 0.5 kJ/mol. We were also able to determine a lower limit of ΔE for salicylic acid, ΔE > 10 kJ/mol. These asymmetrical features in potential energy curves were confirmed by plane-wave DFT computations, which yielded ΔE = 3.7 and 17.8 kJ/mol for Aspirin and salicylic acid, respectively. To complement the solid-state (17)O NMR data, we also obtained solid-state (1)H and (13)C NMR spectra for salicylic acid and Aspirin. Using experimental NMR parameters obtained for all magnetic nuclei present in salicylic acid and Aspirin, we found that plane-wave DFT computations can produce highly accurate NMR parameters in well-defined crystalline organic compounds.

Variable-temperature 17O NMR studies allow quantitative evaluation of molecular dynamics in organic solids

We report a comprehensive variable-temperature solid-state (17)O NMR study of three (17)O-labeled crystalline sulfonic acids: 2-aminoethane-1-sulfonic acid (taurine, T), 3-aminopropane-1-sulfonic acid (homotaurine, HT), and 4-aminobutane-1-sulfonic acid (ABSA). In the solid state, all three compounds exist as zwitterionic structures, NH(3)(+)-R-SO(3)(-), in which the SO(3)(-) group is involved in various degrees of O···H-N hydrogen bonding. High-quality (17)O NMR spectra have been obtained for all three compounds under both static and magic angle spinning (MAS) conditions at 21.1 T, allowing the complete set of (17)O NMR tensor parameters to be measured. Assignment of the observed (17)O NMR parameters to the correct oxygen sites in the crystal lattice was achieved with the aid of DFT calculations. By modeling the temperature dependence of (17)O NMR powder line shapes, we have not only confirmed that the SO(3)(-) groups in these compounds undergo a 3-fold rotational jump mechanism but also extracted the corresponding jump rates (10(2)-10(5) s(-1)) and the associated activation energies (E(a)) for this process (E(a) = 48 ± 7, 42 ± 3, and 45 ± 1 kJ mol(-1) for T, HT, and ABSA, respectively). This is the first time that SO(3)(-) rotational dynamics have been directly probed by solid-state (17)O NMR. Using the experimental activation energies for SO(3)(-) rotation, we were able to evaluate quantitatively the total hydrogen bond energy that each SO(3)(-) group is involved in within the crystal lattice. The activation energies also correlate with calculated rotational energy barriers. This work provides a clear illustration of the utility of solid-state (17)O NMR in quantifying dynamic processes occurring in organic solids. Similar studies applied to selectively (17)O-labeled biomolecules would appear to be very feasible.

Solid-State 17O NMR Spectroscopy of Large Protein–Ligand Complexes†

Solid-state O NMR spectroscopy has attracted considerable attention because of its potential as a new probe of biological structures. 2] One prerequisite for such applications is that the sensitivity of currently available methods for solid-state O NMR spectroscopy is sufficient to allow direct detection of weak O (I = 5/2) NMR signals from a biological macromolecule of significant size. Nearly 20 years ago, Oldfield et al. reported the first set of solid-state O NMR spectra for proteins, [O2]hemoglobin and [ O2]myoglobin, under nonspinning (stationary) conditions in a moderate magnetic field of 8.45 T. However, as the authors noted, the poor signalto-noise ratios obtainable at the time did not permit any detailed spectral analysis. Later, Oldfield and co-workers successfully obtained solid-state O NMR spectra for [CO]myoglobin (16.7 kDa per ligand) under conditions of magic-angle spinning (MAS) at 11.7 T. Several groups have since reported solid-state O NMR spectroscopic studies of membrane-bound peptides. Herein we report the first comprehensive solid-state O MAS NMR spectroscopic study of large protein–ligand complexes with an emphasis on addressing the sensitivity issue. In particular, we used two robust protein–ligand complexes, egg-white avidin–[O2]biotin (64 kDa) and ovotransferrin–Al–[O4]oxalate (80 kDa), as benchmark cases to test the detection limit at a high magnetic field of 21.14 T. We discovered that the O spin–lattice relaxation times (T1) in these solid protein–ligand complexes are typically on the order of several milliseconds; therefore, very rapid data collection is possible. Furthermore, we found that several sensitivity-enhancement methods uniquely suited for halfinteger quadrupolar nuclei, such as double-frequency sweep (DFS), rotor-assisted population transfer (RAPT), and hyperbolic secant (HS) pulses, can be used to obtain highquality solid-state O NMR spectra for large protein–ligand complexes. Avidin is a glycoprotein isolated from hen egg white that forms a tetramer with a total molecular weight of about 64 kDa and can bind biotin molecules with extremely high affinity (Kd = 10 m). The O MAS NMR spectrum of the avidin–[O2]biotin complex exhibits a typical line shape arising from second-order quadrupole interactions (Figure 1).

Resolving Multiple Non-equivalent Metal Sites in Magnesium-Containing Metal–Organic Frameworks by Natural Abundance 25Mg Solid-State NMR Spectroscopy

In a spin: Directly differentiating multiple Mg sites in Mg-containing MOFs by (25)Mg solid-state NMR spectroscopy is very challenging at natural abundance. By performing (25)Mg two-dimensional triple-quantum magic-angle spinning solid-state NMR experiments at a magnetic field of 21.1 T at natural abundance, four non-equivalent Mg sites with very similar local environments in α-Mg(3)(HCOO)(6) were unambiguously resolved (see figure).

(25)Mg Solid-State NMR: A Sensitive Probe of Adsorbing Guest Molecules on a Metal Center in Metal-Organic Framework CPO-27-Mg.

Metal-organic frameworks (MOFs) have excellent adsorption capability. To understand their adsorptive properties requires detailed information on the host-guest interaction. The information on MOF desolvation (or activation) is also crucial because the very first step of many applications requires removal of the solvent molecules occluded inside of the pores. Unfortunately, such information is not always available from powder XRD data. Solid-state NMR is an excellent complementary technique to XRD. CPO-27-Mg is a MOF with unusual adsorption ability. The adsorption involves a direct interaction between Mg and guest species. Herein, we present, for the first time, a natural abundance (25)Mg solid-state NMR study of CPO-27-Mg at an ultrahigh magnetic field of 21.1 T. The results provide new physical insights into the effects of dehydration/rehydration and adsorption of guest species on the Mg local environment.

Solid-State 17 O NMR and Computational Studies of C-Nitrosoarene Compounds

We report the first solid-state (17)O NMR determination of the (17)O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for four (17)O-labeled C-nitrosoarene compounds: p-[(17)O]nitroso-N,N-dimethylaniline ([(17)O]NODMA), SnCl(2)(CH(3))(2)([(17)O]NODMA)(2), ZnCl(2)([(17)O]NODMA)(2), and [(17)O]NODMA.HCl. The (17)O quadrupole coupling constants (C(Q)) observed in these C-nitrosoarene compounds are on the order of 10-15 MHz, among the largest values found to date for organic compounds. The (17)O CS tensor in these compounds exhibits remarkable sensitivity toward the nitroso bonding scheme with the chemical shift anisotropy (delta(11) - delta(33)) ranging from just 350 ppm in [(17)O]NODMA.HCl to over 2800 ppm in [(17)O]NODMA. This latter value is among the largest (17)O chemical shift anisotropies reported in the literature. These extremely anisotropic (17)O NMR interactions make C-nitrosoarene compounds excellent test cases that allow us to assess the detection limit of solid-state (17)O NMR. Our results suggest that, at 21.14 T, solid-state (17)O NMR should be applicable to all oxygen-containing organic functional groups. We also show that density functional theory (DFT) calculations can reproduce reasonably well the experimental (17)O QC and CS tensors for these challenging molecules. By combining quantum chemical calculations with experimental solid-state (17)O NMR results, we are able to determine the (17)O QC and CS tensor orientations in the molecular frame of reference for C-nitrosoarenes. We present a detailed analysis illustrating how magnetic field-induced mixing between individual molecular orbitals (MOs) contributes to the (17)O shielding tensor in C-nitrosoarene compounds. We also perform a Townes-Dailey analysis for the observed (17)O QC tensors and show that (17)O CS and QC tensors are intrinsically related through the pi bond order of the N horizontal lineO bond. Furthermore, we are able for the first time to examine the parallelism between individual (17)O and (15)N CS tensor components in C-nitrosoarenes.

Solid-state ¹⁷O NMR spectroscopy of paramagnetic coordination compounds.

High-quality solid-state (17)O (I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers. For complexes containing V(III) (S=1), Cu(II) (S=1/2), and Mn(III) (S=2) metal centers, the (17)O isotropic paramagnetic shifts were found to span a range of more than 10,000 ppm. In several cases, high-resolution (17)O NMR spectra were recorded under very fast magic-angle spinning (MAS) conditions at 21.1 T. Quantum-chemical computations using density functional theory (DFT) qualitatively reproduced the experimental (17)O hyperfine shift tensors.