125Te NMR tomography of [Re6] cluster in Re6Te15 and related cluster compounds

The static and magic angle spinning NMR solid-state spectra of Re6Te15 and a series of Re(6)-Te cluster compounds demonstrate the positive and negative 125Te NMR chemical shifts indicating that the [Re6] cluster distorts strongly magnetic field at the neighboring Te sites. The local magnetic field is decreased at the facial tellurium sites (Teface) and enhanced at the apexial tellurium sites (Teapex). The facial and sagittal sections of the Re(6)-Te clusters were reconstructed from analysis of Teface and Teapex NMR spectra, respectively.     

13C CP MAS NMR of halogenated (Cl,Br, I) pharmaceuticals at ultrahigh magnetic fields

This work reports significantly improved spectral resolution of ¹³C CP MAS NMR spectra of chlorinated, brominated and iodinated solid organic compounds when such spectra are recorded at ultrahigh magnetic field strengths. The cause of this is the residual dipolar coupling between carbon atoms and quadrupolar halogen nuclides (chlorine‐35/37, bromine‐79/81 or iodine‐127), an effect inversely proportional to the magnetic field strength which declines in importance markedly at 21.1 T as compared to lower fields. In favorable cases, the fine structure observed can be used for spectral assignment, e.g. for Cl‐substituted aromatics where the substituted carbon as well as the ortho‐carbons show distinct doublets. The experimental results presented are supported by theoretical modeling and calculations. The improved spectral resolution in the studied systems and similar halogenated materials will be of particular interest and importance for polymorph identification, drug discovery and quality control in the pharmaceutical industry. Copyright © 2009 John Wiley & Sons, Ltd.     

13C chemical shift tensors in MOF α-Mg3(HCOO)6: Which component is more sensitive to host-guest interaction?

Metal–organic frameworks (MOFs) are a class of important porous materials with many current and potential applications. Their applications almost always involve the interaction between host framework and guest species. Therefore, understanding of host–guest interaction in MOF systems is fundamentally important. Solid-state NMR spectroscopy is an excellent technique for investigating host–guest interaction as it provides information complementary to that obtained from X-ray diffraction. In this work, using MOF α-Mg₃(HCOO)₆ as an example, we demonstrated that ¹³C chemical shift tensor of organic linker can be utilized to probe the host–guest interaction in MOFs. Obtaining ¹³C chemical shift tensor components (δ₁₁, δ₂₂, and δ₃₃, where δ₁₁ ≥ δ₂₂ ≥ δ₃₃) in this MOF is particularly challenging as there are six coordinatively equivalent but crystallographically non-equivalent carbons in the unit cell with very similar local coordination environment. Two-dimensional magic-angle-turning experiments were employed to measure the ¹³C chemical shift tensors of each individual crystallographically non-equivalent carbon in three microporous α-Mg₃(HCOO)₆ samples with different guest species. The results indicate that the δ₂₂ component (with its direction approximately being co-planar with the formate anion and perpendicular to the C−H bond) is more sensitive to the adsorbate molecules inside the MOF channel due to the weak C−H···O hydrogen bonding or the ring current effect of benzene. The ¹³C isotropic chemical shift, on the other hand, seems much less sensitive to the subtle changes in the local environment around formate linker induced by adsorption. The approach described in this study may be used in future studies on host–guest interaction within MOFs.     

Variable-temperature (17)o 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.     

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 ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (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 ⁶⁷Zn 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 ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn 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 ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn 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.