Transient Techniques in NMR of Solids: An Introduction to Theory and Practice

This volume is an ideal starting point for the graduate student seeking a basic introduction to the theory and uses of solid-state nuclear magnetic resonance (NMR) spectroscopy. Accessible to students with only a survey-level physics background, the material assumes little prior knowledge of the basic theory of electromagnetism. All the major areas are covered, including an introduction to concepts of time-dependent quantum mechanics as they apply to NMR spectroscopy of the solid state. Each chapter includes problems designed to enhance the reader's understanding of the material. Instructive and practical, this volume provides the basic knowledge needed to access the general literature and the more advanced monographs on this subject. In addition to assisting entrance into the field, Transient Techniques in NMR of Solids will be a useful guide for professionals already working in related areas of chemistry. FROM THE PREFACE: Nuclear magnetic resonance (NMR) is truly a remarkable phenomenon. Remarkable can imply different things to different people. From the point of view of a physicist, spin dynamics is an elegant example of the use of time-dependent quantum mechanics, and NMR absorption of energy is a prototype for spectroscopic transitions. From the point of view of the practicing chemist and materials scientist, NMR spectroscopy is an invaluable tool for the identification of chemical species and structures. Had NMR spectroscopic techniques commercially available in the early 1960s been the only result of investigations of this phenomenon, it would have had a major impact on the course of chemical analysis. The study of liquids and solutions for chemical shifts and couplings of protons had produced a rapid means of identifying chemical species nondestructively. The study of dynamical properties also could be addressed by study of temperature dependence of the spectra or of the saturation of the resonance by high-power irradiation. Even at that time, however, studies of the spin dynamics had already begun to indicate that there were many interesting facets of the NMR phenomenon left to exploit. For example, the Fourier-transform relationship of the free-induction decay and the absorption spectrum had been shown and the basis of the cross-polarization experiment was being investigated. A number of chemists had begun to study the spin*b1lattice relaxation times of species by pulse NMR techniques by utilizing methods that were not familiar at that time to the typical chemist but that are now commonly employed in NMR analysis. The principal characteristic of the NMR technique that makes it so useful for chemical analysis of liquids and solutions is the high resolution that allows one to observe very small interactions such as the chemical shift and the spin*b1spin coupling. These weak interactions are quite sensitive to the local environment of the spin and therefore may be used as a diagnostic for the environment. The connectivity of chemical structure is often mimicked closely in the NMR connectivity of the spectrum, and quantitative informaton is relatively easy to obtain. Nuclear magnetic resonance spectra of solids exhibit such resolution only in special cases. The primary (although not the exclusive) reason for the lack of resolution in the spectrum of a typical solid is the presence of the dipole*b1dipole interaction, which dominates the NMR spectroscopy of solids that have been of interest to chemists. One solution (no pun intended) to the problem of obtaining chemical-shift information about such solids is to dissolve them and to study them in solution. However, if the solid is insoluble or otherwise intractable or if the analysis involves questions about the properties of the substance in the solid state, then there arises a need for techniques to study the weaker interactions in the presence of the dipole*b1dipole interaction or other overwhelming interactions. This volume describes the means devised by a number of very clever spectroscopists to achieve this goal. Understanding, like remarkable, can imply different things to different people. We have tried to speak to the graduate student who earnestly wishes to learn about NMR spectroscopy of the solid state. A knowledge of quantum mechanics such as one might get in a junior-level physical chemistry course is presumed. Since many graduate students in chemistry characteristically have not been exposed to physics beyond the level of a survey course, very little prior knowledge of the basic theory of electromagnetism is assumed. The reader who is already familiar with NMR of the solid state may thus find that our explanations are long and circuitous. We have strived to provide a background sufficient to allow the student to understand the results at hand. We have also intended this volume as an introduction to concepts of time-dependent quantum mechanics as they apply to NMR spectroscopy of the solid state. As such, we have chosen to discuss certain topics that represent broad applications of this theory to the NMR of solids. Certain other topics, e.g., two-dimensional NMR techniques and multiquantum studies, have been given little exposition here. This choice certainly does not represent any bias on the parts of the authors about the relevance of these techniques to NMR spectroscopy of the solid state. In the sense that we have provided information for the reader that may help to ease the introduction into this subject, we hope we have contributed to a bridging of the gap of understanding that often frustrates the new student of this field when he encounters the literature, the more advanced books, or a recalcitrant spectrometer. It is our hope that after having read this volume, the student will continue studies of more advanced works but that this work will serve as a reference on some of the machinations that NMR spectroscopists use to explain their experiments on solids.