The Permutational Approach To Dynamic Stereochemistry

Stereochemistry is the part of chemistry that relates observable prop erties of chemical compounds to the structure of their molecules, i. e. the relative spatial arrangement of their constituent atoms. In classical stereochemistry, the spatial arrangements relevant for interpreting and predicting a given chemical property are customarily described by geometric features/ symmetries in some suitably chosen rigid model of the molecule The solution of stereochemical problems involving single molecular species is the danain of the geometry based approaches, such as the methods of classical stereochemistry, molecular mechanics and quantum chemistry. The molecules of a pure chemical compound form generally an ensemble of molecular individuals that differ in geometry and energy. Thus it is generally impossible to represent a chemical compund adequately by the geo metry of a rigid molecular model. In modern stereochemistry it is often necessary to analyze molecular relation within ensembles and families of stereoisomers and permutation isomers, including molecules whose geometric features are changing with time. Accordingly, there is definitely a need for new types of ideas, concepts, theories and techniques that are usable beyond the scope of customary methodology. This is why the present text was written.

The progress in computer technology during the last 10-15 years has enabled the performance of ever more precise quantum mechanical calculations related to structure and interactions of chemical compounds. However, the qualitative models relating electronic structure to molecular geometry have not progressed at the same pace. There is a continuing need in chemistry for simple concepts and qualitatively clear pictures that are also quantitatively comparable to ab initio quantum chemical calculations. Topological methods and, more specifically, graph theory as a fixed-point topology, provide in principle a chance to fill this gap. With its more than 100 years of applications to chemistry, graph theory has proven to be of vital importance as the most natural language of chemistry. The explosive development of chemical graph theory during the last 20 years has increasingly overlapped with quantum chemistry. Besides contributing to the solution of various problems in theoretical chemistry, this development indicates that topology is an underlying principle that explains the success of quantum mechanics and goes beyond it, thus promising to bear more fruit in the future.

This volume contains a selection of scientific papers related to the structure and dynamics of non-rigid molecules. This frontline topic was born a few decades ago, when Longuet-Higgins proposed his famous theory of Molecular Symmetry Groups (Mol. Phys. 6, (1962) 457). Unfortunately, since this early paper, very few publications have been devoted to the study of non-rigid molecules. Let us mention some books which dedicate some chapters to them: Induced Representations in Crystals and Molecules, by S. L. Altmann, Academic Publishers, 1977; Molecular Symmetry and Spectroscopy, by P. R. Bunker, Academic Publishers, 1979; and finally Large Amplitude Motion in Molecules, Vols. I and II, by several authors, Springer Verlag, 1979. More recently an International Symposium on Non-Rigid Molecules was held in Paris, France, from 1-7 July 1982, the proceedings of which were published in the volume entitled Symmetries and Properties of Non-Rigid Molecules. A Comprehensive Survey, edited by J. Maruani et al., Elsevier, 1983. Finally, we should mention the very specialized work The Permutational Approach to Dynamic Stereochemistry, by J. Brocas et al., McGraw-Hill, 1983. The purpose of this book is to fill in this information on the structure and dynamics of non-rigid systems. To this aim, we have gathered a collection of recent papers written by the most qualified specialists in the world, covering a large field from van der Waals molecules to inorganic complexes and organic polyrotor molecules, as well as considering statistical and dynamic aspects.

so easy it seemed Once found, which yet unfound most would have thought Impossible. (John Milton, 1608 -1674) There are essentially two types of books on a scientific subject: in the first one several authors contribute their specialized approaches to parts of the field in question, which then are edited and compiled to yield a comprehensive and authoritative account. In the second type of book a single author tries to pre sent a view from an individual standpoint which might lead to a more balanced and homogeneous source of information. Both kinds have their merits and de ficiencies. I decided to write this book as a monolithic piece of work for several rea sons. Of course, there was the challenge of coping with the many problems of such an undertaking due to the fact that this field has grown tremendously during the last decades. In addition, being heavily involved in linear oligopyr role chemistry for nearly two decades, it seemed worthwile to prepare a more unifying approach. The request of several colleagues from abroad to give an account in English also triggered this endeavor since most of the work of my group has been published in German.

Each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole. The most significant developments of the last 5 to 10 years are presented using selected examples to illustrate the principles discussed. The coverage is not intended to be an exhaustive summary of the field or include large quantities of data, but should rather be conceptual, concentrating on the methodological thinking that will allow the non-specialist reader to understand the information presented. Contributions also offer an outlook on potential future developments in the field.