Understanding hydrogen-bonding networks in nanocrystals and microcrystals that are too small for X-ray diffractometry is a challenge. Although electron diffraction (ED) or electron 3D crystallography are applicable to determining the structures of such nanocrystals owing to their strong scattering power, these techniques still lead to ambiguities in the hydrogen atom positions and misassignments of atoms with similar atomic numbers such as carbon, nitrogen, and oxygen. Here, we propose a technique combining ED, solid-state NMR (SSNMR), and first-principles quantum calculations to overcome these limitations. The rotational ED method is first used to determine the positions of the non-hydrogen atoms, and SSNMR is then applied to ascertain the hydrogen atom positions and assign the carbon, nitrogen, and oxygen atoms via the NMR signals for 1 H, 13 C, 14 N, and 15 N with the aid of quantum computations. This approach elucidates the hydrogen-bonding networks in l -histidine and cimetidine form B whose structure was previously unknown.

The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed “too challenging”.

A protocol for the detection by NMR spectroscopy of trace amounts of quartz in amorphous silica gels was developed and tested on commercially available samples. Using 29 Si MAS NMR spectroscopy with CPMG acquisition and standard addition of crystalline quartz, quantitative detection of quartz concentrations down to 0.1 %wt. was achieved. CPMG permitted to suppress the amorphous silica derived signal, benefitting from the extremely long T2 relaxation time of quartz in 29 Si, and hence dramatically increasing the sensitivity. Dedicated post-processing exploiting the known CPMG spikelet frequencies allowed to probe the nearabsence of quartz in commercial, 100 % silica samples, enabling to assess conformity of unknown samples to EU legislation (REACH).Amorphous silica is commonly used in large scale applications: as desiccant, as filler in elastomers, for controlled release of drugs or as component in pest control systems. [1][2][3][4][5][6][7] Quartz and other forms of respirable crystalline silica (RCS), on the contrary, are known to cause silicosis, lung cancer and chronic obstructive pulmonary disease (COPD). 8-10 European Union regulations [REACH], require silica gel manufacturers to certify the absence of crystalline SiO2 in their products, defining a maximum allowed concentration of 0.1 %wt. The most common method to assess crystalline material in an otherwise amorphous sample is X-ray diffraction. Methods used to determine the quartz /cristobalite content in dust, collected in workplace atmospheres, are Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). 11,12 Reference standards for both methods are described in:

No abstract

Overlapping (13)C or (15)N solid-state NMR spectra from crystallographically different forms of L-arginine hydrochloride can be separated by exploiting differential proton T(1) relaxation in conjunction with cross-polarization. Dipolar (13)C-(13)C and (15)N-(15)N two-dimensional correlation experiments reveal resonances belonging to crystallographically and magnetically inequivalent molecules.