Nuclear magnetic relaxation, the evolution from non-equilibrium spin populations and coherences to thermal equilibrium, is ubiquitous in nuclear magnetic resonance (NMR). All NMR experiments, including spectroscopy, are fundamentally based on the generation of non-equilibrium spin states. Otherwise, spin resonance phenomena would not be detectable. In this sense, this book does not cover all aspects of relaxation in NMR in general. Rather, the goal is to outline the unique potential of true NMR relaxation experiments to elucidate molecular dynamics. The key concepts discussed in the following chapters are correlation functions and spectral densities of fluctuations of spin interactions as a consequence of thermal molecular processes. These are the leading carriers of information about all features of molecular dynamics. NMR relaxometry provides an exceptional set of tools to access these features directly and in a wide range of system variables.

There are excellent books, book chapters or review articles dealing with nuclear magnetic relaxation. These accounts often include elaborate and comprehensive spin dynamics formalisms and nuclear magnetic relaxation theories. In contrast, descriptions of the statistics of molecular dynamics usually do not go beyond typical textbook models, such as isotropic rotational diffusion. This book takes a somewhat different approach: users studying nuclear magnetic relaxation in media, including fluids in porous materials, polymers, liquid crystals, biological tissues or colloids, need tools for developing statistical models for the rather complex molecular dynamics to be expected in such systems. In many, if not most, cases the textbook scenario of rotationally diffusing molecules with Markovian statistics is no longer adequate to describe Brownian motion. Instead, processes involving molecular reorientations conditionally combined with translational diffusion, exchange between different molecular environments and collective reorientations and translations are likely to be crucial. Moreover, the complexity of the dominant processes often manifests itself in non-Markovian statistics.

In this sense, the theory of spin relaxation, on the one hand, and the statistical description of molecular motion, on the other hand, should be considered as two independent and separate problem areas. The emphasis of this book is placed on molecular dynamics rather than on spin dynamics. The author has therefore restricted himself to the presentation of the familiar perturbation-theoretical relaxation formalism valid in the high-field limit of the quantizing field.

The interpretation of experimental data in terms of molecular dynamics models is a crucial and final step in NMR relaxation studies. The exemplary model theories presented in the application chapters of this book are intended as inspirational aids. In each case, there may be competing concepts that could be the subject of alternative considerations. In ambiguous cases, it is ultimately the responsibility of the researcher to find arguments for one view or the other. In this sense, the present text may help users of NMR relaxometry to identify critical and decisive clues. May this book inspire the interested reader to use NMR relaxation tools without becoming lost in abstract spin formalisms, on the one hand, and without getting stranded in experimental artifacts, on the other.

The book is structured in a ‘problem-oriented’ approach. Readers looking for the relaxation method best suited for an upcoming research project will find a chapter with a compilation of all common NMR relaxation methods. Those who are specifically interested in the relevant spin interactions, or in high-field relaxation phenomena based on them, can step into corresponding recapitulatory chapters. Other chapters are devoted to statistical aspects of molecular dynamics and exchange characteristics. In the last part of the book, there are a number of descriptions of practical research problems, which are outlined along with experimental datasets and theoretical interpretations. With this structural design, the author hopes that he is offering readers the option to conveniently and straightforwardly access the topic of interest at the level of prior knowledge.

The contents of this book largely reflect the experiences and insights gained by the author over many years of research and academic teaching. He would like to thank all those involved, whether students, postdocs or colleagues, for their cooperation, support, and friendship. Thanks are also due to the RSC Books Team for their excellent cooperation in the preparation of the manuscript.

Rainer Kimmich

Ulm