Concepts in Magnetic Resonance Part A最新文献

Metal-organic frameworks (MOFs) are exciting porous materials with a growing number of applications ranging from catalysis to gas storage. Establishing logical connections between the local MOF structure and its properties is not often straightforward, however, solid-state NMR is a sensitive probe of local structure and can be used to shed light on processes such as guest adsorption and gas motion within MOFs. As illustrated using our recent works on the microporous α-Mg-formate (Mg₃(HCOO)₆) MOF, complete multinuclear solid-state NMR characterization of MOFs is now possible, and can provide unique insight that is not readily available through other methods. A wide variety of solid-state NMR techniques have been employed, including direct-excitation, cross-polarization, fast magic-angle spinning, and two-dimensional experiments. New variable-temperature ²H solid-state NMR data of deuterated hydrogen gas within α-Mg-formate and the resulting detailed dynamic information is also presented, analyzed, and discussed.

The multi-echo spin-echo sequence is a series of operators, referred to as periodic operators. Each periodic operator consists of a free rotation (no RF), a refocusing RF pulse, and another free rotation, identical to the first one. A preparation operator that precedes the periodic operators converts the equilibrium magnetization M_z into an initial magnetization M_i. It is shown that a multi-echo sequence is equivalent to a simple rotation of the magnetization about a tilted axis. The component of M_i along the rotation axis is stationary and provides a stable signal, denoted pseudo steady-state. The perpendicular component rotates and eventually de-phases. Using this model, we derive analytic expressions to the signal for different preparation operators, and show how to align M_i with the rotation axis such that the signal is maximized. A simple and efficient algorithm is presented to calculate the Fourier coefficients of the magnetization during the sequence using the discrete Fourier transform. Finally, formulas of the echo signal when unavoidable phase errors are generated are derived. We show how to eliminate artifacts caused by these errors and restore the original image.

We report a solid-state ¹⁷O (I = 5/2) NMR study of the nitrite ion dynamics in crystalline NaNO₂. Variable temperature (VT) ¹⁷O NMR spectra were recorded at 3 magnetic fields, 11.7, 14.1, and 21.1 T. The VT ¹⁷O NMR data suggest that the ion in the ferroelectric phase of NaNO₂ undergoes 2-fold flip motion about the crystallographic b axis and the corresponding rotational barrier is 68 ± 5 kJ mol⁻¹. We also obtained a 2D ¹⁷O EXSY spectrum for a stationary sample of NaNO₂ at 250 K, which, in combination with 1D ¹⁷O NMR spectral analyses, allowed precise determination of the relative orientation between the ¹⁷O quadrupolar coupling and chemical shift tensors in the molecular frame of reference. The experimentally determined ¹⁷O NMR tensors for NaNO₂ were in agreement with quantum chemical calculations produced by a periodic DFT code BAND.

The structures of many small molecules have been determined using residual dipolar couplings (RDCs) and the singular value decomposition method (SVD). Here, a key part of the SVD method is examined. The methods used to predict the 3-dimensional structures of molecules used in the SVD methodology for comparison with RDCs were examined for their impact on the quality of agreement that those structures had with observed RDCs. It was observed that the method of generating a proposed structure had a great impact on the quality of agreement with observed spectral data and that more computationally intensive methods of proposing structures provided better agreement with the observed RDCs in the control molecule.