Preface Free

Over recent years, there has been a growing interest in molecular chirality in the scientific community. This growing interest is partly driven by the development of new research techniques, theoretical descriptions of molecules and the discovery of new systems for which the understanding of chirality is fundamentally important. The examples include studies of chirality-induced spin selectivity phenomena, chiral single-molecule magnets, the application of X-ray lasers for chirality studies, and the growing percentage of chiral substances among active pharmaceutical ingredients.

How does nuclear magnetic resonance (NMR) spectroscopy fit into the area of such lively developing studies of molecular chirality? On the one hand, NMR provides a lot of information about molecular structures with atomic resolution. On the other hand, in contrast to X-ray, UV-Vis, and microwave branches of spectroscopy, it is inherently insensitive to chirality in a direct way. Therefore, NMR spectroscopists are exploring the effects of introducing different types of chiral environments in which molecules are placed. The effects of such environments on an observed nucleus may be a consequence of the chemical modification of a molecule or the usage of a chiral solvent. This is one of the main topics of this book. Another focus is exploring new, unexplored areas of research on chirality using NMR. An example is a constant and wide-ranging search for new NMR observables that are directly sensitive to molecular chirality. The possibility of making NMR spectroscopy chirality-sensitive is deeply related to the symmetry inherent in the fundamental laws of Nature. This issue arises from the study of the interactions of elementary particles, and this book covers it to the extent that it helps understand chirality in chemistry.

The information needed to understand chirality in the context of NMR spectroscopy comes from many, sometimes distant, fields of science. This diversity is reflected in the level of the technical language used in different parts of the book. For readers more focused on practical applications, we have placed emphasis on pointing out the sources of a given research method rather than providing a detailed and complete description. Readers who are more inclined towards exploring the physics behind chirality-sensitive phenomena will find the relevant details in the cited literature.

The book content, in brief, is as follows. Chapter 1 provides an overview of the basics of chirality and focuses on answering the question of what chirality is and how it manifests itself in molecules. Chapter 2 discusses tools for the study of chiral molecules that have been developed by spectroscopists, considered from a broader perspective that extends beyond NMR spectroscopy, to give readers a full overview of the methods also used in cases where alternative methods to NMR spectroscopy can provide valuable data about the experimental system under study. Then, in Chapter 3, the term chirality is discussed from the perspective of a mathematician. This chapter also highlights that chirality is a gradable concept, and depending on the context, there are stronger and weaker variants of chirality, which is important as we move from the analysis of rigid molecules to flexible molecules, which constitute the majority of known molecules to chemists. Chapters 4 and 5 describe the classic indirect NMR methods for the study of chiral compounds with particular emphasis on the physical basis of these methods and provide an overview of the chiral reagents to facilitate the selection of a specific agent for a given molecule based on its chemical groups. In Chapter 6, the relationship between nuclear magnetic shielding and chirality is examined and described in the case of probing chirality using a rare gas, xenon, introduced into a molecule having a chiral cage – cryptophane. In Chapters 7 and 8, the basics of nuclear magnetoelectric resonance (NMER) spectroscopy are given, including the inherent relationship between antisymmetric nuclear interactions and molecular chirality, several examples illustrating how the molecular structure and symmetry relate to antisymmetric properties that may at first seem to have no intuitive relation to the shape of the particle, detailed description of the experimental protocols and in-depth consideration of required experimental setups. Chapter 9 describes a modification of the NMR experiment in which, instead of resonance conditions, applying a transient wave permits one to discriminate directly between enantiomers. Finally, Chapter 10 addresses the issue of relatively low sensitivity of NMR compared to optical spectroscopy methods and shows how hyperpolarization techniques can be used to enhance chirality-sensitive signals in NMR spectroscopy.

We hope that Physical Principles of Chirality in NMR will provide a reader-friendly overview of both the currently known classical indirect methods of studying chirality and those still under development, along with those that will be able to enrich the wide range of information provided by NMR with direct chirality determination in the future.

Piotr Garbacz

University of Warsaw, Poland