14N solid-state NMR is useful for differentiating polymorphs and chemically distinct nitrogen-containing compounds. A case study of glycine is presented.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, (14)N and (15)N, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality (14)N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen-containing systems. We demonstrate that it is possible to dramatically enhance (14)N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., (1)H). The BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) pulse sequence is combined with other elements for efficient acquisition of ultra-wideline SSNMR spectra, including Wideband Uniform-Rate Smooth-Truncation (WURST) pulses for broadband refocusing, Carr-Purcell Meiboom-Gill (CPMG) echo trains for T2-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for (14)N, with special consideration given to (i) spin-locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN-CP experiments are shown to provide increases in signal-to-noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where (14)N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments.

No abstract

Many NMR-active nuclei give rise to solid-state NMR spectra that span extremely large frequency regions due to the effects of large anisotropic NMR interactions; such spectra, which can range from 250 kHz to several MHz in breadth, have been termed ultrawideline (UW) NMR spectra. UWNMR spectra are often too broad to be uniformly excited by conventional pulse sequences that implement rectangular radiofrequency (RF) pulses. Therefore, they are typically acquired with specialized pulse sequences and frequencyswept (FS) pulses; however, such experiments are conducted predominantly upon stationary samples (i.e., static NMR with no magic-angle spinning, MAS). Herein, we demonstrate how to implement Carr−Purcell Meiboom−Gill (CPMG) type pulse sequences that utilize rectangular pulses to acquire high-quality wideline and UWNMR spectra under MAS conditions, which are useful for providing uniformly excited patterns with substantial signal enhancements in comparison to conventional MAS NMR spectra and identifying peaks and/or patterns corresponding to magnetically nonequivalent sites. We discuss the pulse timings, delays, and the duration of windowed acquisition periods that are necessary for using CPMG-type pulse sequences for T 2 -dependent enhancement under MAS conditions while allowing for chemical shift resolution and maintaining a conventional spinning-sideband (SSB) manifold, as well as protocols for processing these spectra. Careful consideration is given to pulse lengths and RF amplitudes used in these pulse sequences. Using several spin-1 / 2 (i.e., 119 Sn, 207 Pb, 195 Pt) nuclei and one integer-spin quadrupolar nucleus (i.e., 2 H), we show how sensitivity-enhancing protocols, including CPMG and cross-polarization (CP), can be used to deliver high-quality MAS NMR spectra, which feature high signal-to-noise (S/N) ratios and uniformly excited SSB manifolds. The methods outlined herein are facile to implement and offer the potential to open up MAS NMR experiments to a wide variety of elements from across the periodic table.

N ultra-wideline solid-state NMR (SSNMR) spectra were obtained for 16 naturally occurring amino acids and four related derivatives by using the WURST-CPMG (wideband, uniform rate, and smooth truncation Carr-Purcell-Meiboom-Gill) pulse sequence and frequency-stepped techniques. The N quadrupolar parameters were measured for the sp nitrogen moieties (quadrupolar coupling constant, C , values ranged from 0.8 to 1.5 MHz). With the aid of plane-wave DFT calculations of the N electric-field gradient tensor parameters and orientations, the moieties were grouped into three categories according to the values of the quadrupolar asymmetry parameter, η : low (≤0.3), intermediate (0.31-0.7), and high (≥0.71). For RNH moieties, greater variation in N-H bond lengths was observed for systems with intermediate η values than for those with low η values (this variation arose from different intermolecular hydrogen-bonding arrangements). Strategies for increasing the efficiency of N SSNMR spectroscopy experiments were discussed, including the use of sample deuteration, high-power H decoupling, processing strategies, high magnetic fields, and broadband cross-polarization (BRAIN-CP). The temperature-dependent rotations of the NH groups and their influence on N transverse relaxation rates were examined. Finally, N SSNMR spectroscopy was used to differentiate two polymorphs of l-histidine through their quadrupolar parameters and transverse relaxation time constants. The strategies outlined herein permitted the rapid acquisition of directly detected N SSNMR spectra that to date was not matched by other proposed methods.

A hydrogen-bonded solid has pores sustained by complementary interactions between hexaaqua metal cations and phosphonate anions. The pores are templated by guest molecules of a certain size and chemical functionality, but these guests are removable. Even though the activated form of the solid is 27% water by weight, reversible gas sorption is demonstrated. The framework shows adaptability to different guest species.

NMR crystallography is an emerging discipline that combines solid-state NMR (SSNMR) spectroscopy, X-ray diffraction (XRD) methods, and computational approaches for the purposes of refining and determining molecular-level structures in a wide array of solids, including crystalline, semi-ordered, and amorphous materials.[1-3] SSNMR can be utilized to provide information on interatomic distances, structural assignments, local atomic/molecular symmetries, and/or characterization of structural disorder; these data, when used in combination with XRD and/or computational methods, can elicit structures that rival those determined by neutron diffraction methods. The majority of modern NMR crystallographic studies rely upon the measurement of chemical shifts (typically from 1 H, 13 C, or 15 N NMR spectra), and comparison to magnetic shielding values of refined structures obtained from plane-wave density functional theory (DFT) calculations. An increasing number of studies have utilized data from numerous NMR-active nuclides across the periodic table, including metal nuclides with large chemical shift anisotropies and quadrupolar nuclides (i.e., nuclear spin > 1/2). Quadrupolar nuclides are of particular interest, since the quadrupolar interactions that influence SSNMR spectra are extremely sensitive to even the smallest structural differences/changes. In this lecture, first, I will present a discussion of NMR crystallographic studies conducted in my group, with a focus on structural refinements aided by 14 N, 17 O, 35 Cl, 111 Cd and 195 Pt solid-state NMR data. These nuclides are can be classified as unreceptive, due to a number of factors, including: (i) low gyromagnetic ratios, (ii) low natural abundance, (iii) large anisotropic interactions that can lead to substantial line broadening, (iv) inconvenient relaxation characteristics, or (v) combinations of these factors. Then, I will discuss some of the methods designed by my group that allow for rapid acquisition of SSNMR spectra crucial for NMR crystallographic studies.[5] Finally, I will outline a powerful method for refining crystal structures that uses dispersion-corrected plane-wave DFT, which relies upon the accurate measurement and computation of electric field gradient (EFG) tensors.[6]