Solid-state NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy has come to occupy a central role in chemistry since its first demonstration in condensed phases in the mid-1940s. In the solution state, fast molecular tumbling reduces the NMR interactions to simple isotropic averages, resulting in readily interpretable spectra. In contrast, the impact of solid-state NMR has been less immediate. This largely reflects the more complex nature of the nuclear spin Hamiltonian in the absence of rapid tumbling. Unsophisticated NMR on solids results in broad, relatively uninformative spectra.

Recent years have, however, seen a quiet revolution in solid-state NMR. The availability of superconducting magnets operating at very high magnetic fields (in excess of 20 T), together with magic angle spinning NMR probes capable of spinning samples at rates of up to 70 kHz, and advances in experimental methodology have transformed the resolving power and sensitivity of solid-state NMR. This has been particularly important for “quadrupolar” nuclei (with spin quantum number I > 1/2) where resolution in the solution-state is often severely limited by motionally-driven quadrupolar relaxation. Improved resolution allows J couplings to be routinely observed, allowing a wide range of two-dimensional correlation methods to be brought over from solution-state NMR. These developments are highly significant as many of the major challenges facing the chemical sciences involve complex materials for which the ability of NMR to probe local chemical environment is a key asset. The expansion of solid-state NMR infrastructure, in specialist research groups, institutional services, and, increasingly, very high field national and international facilities, has made these advances widely available.

Although NMR parameters are highly sensitive to local environment, dynamics and disorder, the lack of a direct relationship between experimental data and the solid-state structure has been a major drawback of the technique in comparison to diffraction-based methods. However, the advent of efficient density functional theory (DFT) methods that utilize periodic boundary conditions means it is now possible to calculate NMR parameters such as the chemical shift and quadrupolar coupling in the crystalline state with good accuracy. This allows a much more direct connection to be made between the NMR parameters and structure, transforming the information content of NMR spectra and making NMR an ideal complement to methods probing long-range order.

This special issue begins with two Perspectives articles. Lesage reviews recent developments in solid-state NMR spectroscopy of spin I = 1/2 nuclei such as ¹H and ¹³C, concentrating on new methods for obtaining high-resolution spectra of abundant nuclei (e.g.¹H or ¹⁹F) and on two-dimensional correlation techniques for exploiting scalar and dipolar couplings. In the second Perspectives article, Ashbrook introduces solid-state NMR of quadrupolar nuclei, and discusses methods of obtaining high-resolution spectra, including from “difficult” nuclei with low gyromagnetic ratios.

The research articles in this issue showcase the current state of solid-state NMR. The articles from Titman and L. Mueller show how novel RF pulse sequences can be used to manipulate the nuclear spin response to pull out different information, while other articles emphasise the role of experimental hardware in exploring new spin physics (Hayes) or simplifying the NMR spectrum (Dupree). The availability of higher magnetic fields and new experimental methods has allowed previously difficult NMR nuclei to be exploited, such as ^35/37Cl (Bryce), ³³S (Jakobsen), ²⁵Mg (K. Mueller) and ¹⁴N (Schurko), while computational chemistry has an increasingly important role in connecting experiment and structure (Bryce, Bonhomme, Schurko).

The ability of NMR to probe local environment independently of physical state means that the applications of solid-state NMR are wide ranging. Not only can NMR be applied to relatively ordered organic solids (Brown, Wu), but also materials with considerable disorder (Deschamps, Dupree), including polymeric materials (Ries, Kroeker). Even surface-absorbed species affected by both “static” and dynamic disorder may be amenable to NMR (Emsley). The sensitivity of different NMR parameters to motions on a wide variety of timescales from nanoseconds (spin–lattice relaxation) to seconds (“chemical exchange”) makes NMR an ideal tool for characterising dynamic processes in solids (Reichert, Ries, Bonhomme, Emsley, Saalwächter, Vold). While the presence of paramagnetism can limit the information content of NMR spectra, solid-state NMR is often perfectly viable in paramagnetic materials, as illustrated in the articles from Stebbins and Kroeker. Finally, many of the recent developments in solid-state NMR have been driven by potential applications to complex biological solids that cannot be studied by Bragg diffraction methods, such as intact membrane proteins or amyloid fibrils. Although the applications to these biological systems fall outside the scope of this journal, this issue includes articles from L. Mueller and Ulrich concerned with methodological developments for the NMR of solid or membrane-bound proteins.

We hope that this survey of solid-state NMR and its applications will be a valuable reference not only for subject experts, but also those wishing to explore what this powerful, but subtle technique has to offer. We thank the authors for submitting a high quality set of articles on time, and thank the PCCP staff and reviewers for their efficient handling of the sometimes fraught reviewing process. Finally, we thank Professor Robin Harris for his original proposal for this issue.

Paul Hodgkinson, Durham, UK

Stephen Wimperis, Glasgow, UK

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