Detection and use of small J couplings in solid state NMR experiments

Abstract

Over the years, solid-state nuclear magnetic resonance (NMR) spectroscopy has become an important tool for materials science, with its local point of view that is highly complementary to the structural information provided by diffraction techniques, electron microscopy, and molecular modeling, for example. As compared to other interactions that determine the spectral expression of the local structure of the observed nuclei in solid-state NMR experiments, the J coupling, characteristic of the chemical bonds, has received far less attention because of its being generally so small that it is masked in the line-widths. Nevertheless, the scalar or isotropic part of J couplings, which is not averaged by magic angle spinning (MAS), can be evidenced in many systems, and exploited to unequivocally characterize the extended coordination sphere. In a first step we describe the different experiments that permit the observation and the measurement of J couplings, even when dealing with quadrupolar nuclei. We then present new and recently-published results that illustrate the state of the art of NMR methodologies based on or intended for measuring J couplings in solids and the novel perspectives that they open towards better understanding of ordered and disordered materials at the subnanometric scale, a length scale that is otherwise difficult to access.

Identification of solvated species present in concentrated and dilute sodium silicate solutions by combined 29Si NMR and SAXS studies

“…This experiment can be applied to liquids [22] or solids under rapid magic angle spinning [23,24]. The timing of the experiment was optimized on the first slice (28 ms pumping time and 28 ms converting time) to obtain the better signal to noise ratio and is a compromise between the generation of double quantum coherences and relaxation times.…”

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

“…5 of 30 Ref. 16 for 1 H and 31 P spin-echo MAS NMR experiments, respectively. To achieve a realistic modelling of the experimental results, it is necessary to consider a model system that additionally takes into account the noncommutation of multiple dipolar couplings.…”

“…There is a growing literature of examples where homonuclear J couplings are determined from spin-echo MAS NMR experiments for spin I = 1/2 nuclei such as 13 C, 15 N, 29 Si and 31 P. 10,16,47,61,[93][94][95][96][97][98] In these cases, cosine modulation due to a J 65 coupling leads to clear zero crossings (at  = n/2J, where n = 1, 3,… for a 90  /2  180  /2  t acq pulse sequence) allowing the J coupling constants to be determined to a high accuracy. While there is still significant signal intensity for the B3 site at a spin-echo duration of 166.6 ms for the 11 B 70 MAS NMR spin-echo data in Fig.…”

Synthesis of Mg2C: A Magnesium Methanide

“…The absence of J coupling in the observed NMR spectrum suggests that carbon within the compound is primarily ionic; increased covalency would be manifested by the presence of J coupling. [19] Similar to other known magnesium carbides, the Mg 2 C compound is easily hydrolyzed by the moisture in air. Samples must be handled cautiously within an inert atmosphere to avoid hydrolysis.…”