Solid‐State Nuclear Magnetic Resonance

The first experimental verification of nuclear magnetic resonance (NMR) in the solid state was carried out by Edward M. Purcell, Henry C. Torrey, and Robert V. Pound in 1945. Since then, new developments have transformed this form of spectroscopy from an interesting physical curiosity at its inception to one of the most widely used analytical tools for physical, chemical, biological, and materials research and applications. The capabilities of NMR include the following: identification of molecules in all phases of matter by their atomic connectivities; probing the temperature-dependent conformation of molecules from cyclohexane to complex 50 kDa proteins; molecular dynamics investigations from the picosecond timescale for rotation of small molecules to the second timescale for atoms diffusing throughout the three-dimensional nanoscopic channels of zeolites; elucidation of chemical reaction mechanisms; the determination of organization, reorientation, and diffusion of molecular solids; the identification of phase changes and morphology of solids; the measurement of bond lengths and interatomic distances between nonbonded atoms; the assessment of pore size in nanoporous solids as well as the identification of adsorbates and their diffusivities; the estimation of physicochemical parameters of molecular systems such as equilibrium and kinetic rate constants, enthalpies and entropies of a reaction, and activation energies; the capability to follow a metabolic pathway taking place inside living cells; the quantification of the alignment of molecules in partially ordered systems; the determination of nanoscale domain sizes in heterogeneous materials; and the noninvasive imaging of all phases and forms of matter under various temperature, pressure, and mechanical conditions. The scope of this article is very selective, covering some of the most widely used techniques and more unusual areas of application of solid-state NMR. These topics are covered at different levels of detail, and some are just mentioned by reference to published work. This article is not intended to present a comprehensive review of all state-of-the-art NMR techniques; for these, the reader should consult references included in the text. Instead, this article provides an historical background and the fundamental concepts of magnetic resonance that preceded solid-state NMR, a general overview of the basic principles with emphasis on the visualization of anisotropic spin interactions, a review of the most widely used methods for investigating solids by NMR, and practical information to provide a modicum of guidance for application of the methods. There are several redundancies in terminology so that different sections can be read independently. The references are an important component of the article and should be consulted for more complete and often original accounts of the theory and techniques for the application of NMR to solids. A modest attempt has been made to include original and important references, but more importantly, many have been included to provide examples of chemical and physical insights obtained in whole or in part by application of NMR techniques to the investigation of the problem. For figures and tables, if no reference or acknowledgment has been provided, the spectrum or photograph in the figure or the table is previously unpublished. The article is directed at a general audience of chemical and physical analytical scientists with an interest in NMR. In addition, NMR spectroscopists may find some interesting details of studies on metals and magnetic materials. Keywords: solid; tensor; induced magnetic field; resonance surface; magic angle spinning; anisotropy; spin relaxation; shielding; Knight shift; quadrupole moment; Rabi field; molecular beam; T-field