Preface

"High resolution NMR in solids" has become the standard term for a variety of experimental techniques which allow the observation in solids of resolved, "chemically shifted" NMR lines from spin-^ nuclei in magnetically equivalent lattice positions. This is usually accomplished at a level of spectral resolution which may seem ridiculously poor by the standards of high resolution NMR proper. But this shortcoming of high resolution NMR in solids is compensated for by "new" information accessible to the experimenter; the most important one concerns nuclear magnetic shielding tensors. A substantial part of this volume is consequently devoted to the determination of nuclear magnetic shielding tensors (Chapters III and VI). Four main approaches to high resolution NMR in solids are currently in use: magic-angle sample-spinning, multiple-pulse, proton-enhanced nuclear induction, and indirect detection methods. They have been tailored to suit a variety of different experimental situations. In this volume we discuss the principles of how "high resolution" is achieved for all of them (Chapter IV). The latter two also involve, as well as means for obtaining "high resolution," ingenious tricks for enhancing the sensitivity of detecting the NMR signal; but we consider these as being outside the scope of this volume. Experimental and theoretical details as well as a comprehensive review of applications are restricted to multiple-pulse techniques (Chapters V and VI). It is in this field that 1 have concentrated my own research efforts in recent years. The leitmotiv of high resolution NMR—generally, not only in solids— is always some kind of selective averaging. By this we mean that those interactions of the nuclear spins which are considered uninteresting in a particular experiment are somehow made time dependent. Ways to accomplish this range from simply melting the sample to applying highly complex multiple-pulse sequences. Provided the time dependences so introduced meet certain conditions, the unwanted spin interactions are efficiently averaged out, whereas the interesting ones remain more or less unaffected. In order to understand what is possible and what is not by selective averaging, a good grasp of the tensorial properties of nuclear spin interactions in both ordinary and spin spaces is required. Chapter II is devoted to a study of these properties. Chapter III deals with the manifestations of nuclear magnetic shielding in NMR spectra of both single-crystal and powder samples. The techniques for analyzing spectra and "rotation patterns" in terms of shielding tensors are discussed. Line broadening by spin-spin interactions is largely disregarded in this chapter.