Atomic Hypothesis and the Concept of Molecular Structure PDF
Preview Atomic Hypothesis and the Concept of Molecular Structure
Theoretical Models of Chemical Bonding Part 1 Atomic Hypothesis and the Concept of Molecular Structure Editor: Z. B. Maksic With contributions by L. D. Barron, 1. E. Boggs, 1. P. Dahl, Z. B. Maksic, A. Y Meyer, O. E. Polansky, B. T. Sutcliffe, K.B. Wiberg With 40 Figures and 51 Tables Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Professor Dr. Zvonimir B. Maksic Theoretical Chemistry Group The "Rudjer Boskovic" Institute 41001 Zagreb, Bijenicka 54/Croatia/Yugoslavia and Faculty of Natural Sciences and Mathematics University of Zagreb 41000 Zagreb, Marnhiev trg 19, Croatia/Yugoslavia Library of Congress Cataloging-in-Publication Data. Atomic hypothesis and the concept of molecular structure / editor, Z. B. Maksie ; with contributions by L. D. Barron ... fet al.l. p. em. - (Theoretical models of chemical bonding; pt. 1) ISBN-13: 978-3-642-64775-8 e-1SBN-13: 978-3-642-61279-4 001: 10.1007/978-3-642-61279-4 I. Molecular structure. 2. Atomic theory. I. Maksie, Z. B. (Zvonimir B.) II. Barron, 1. D. III. Series. QD461.A858 1990 541.2'2-dc20 90-9443 CIP This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, repriuting, reuse of illus trations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions of the German Copyright Law of September 9, 1965, in its current version, and a copyright fee must always be paid. © Springer-Verlag Berlin Heidelberg 1990 Softcover reprint of the hardcover 1s t edition 1990 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement. that such names are exempt from the relevant protective laws and regulations and therefore free for general use. 2152/3020-543210 - Printed on acid-free paper To the memory ofm y parents Olivera and Branko Maksic Series Preface "Imagination and shrewd guesswork are powerful instruments for acquiring scientific knowledge ... " 1. H. van't Hoff The last decades have witnessed a rapid growth of quantum chemistry and a tremendous increase in the number of very accurate ab initio calculations of the electronic structure of molecules yielding results of admirable accuracy. This dramatic progress has opened a new stage in the quantum mechanical description of matter at the molecular level. In the first place, highly accurate results provide severe tests of the quantum mecha nics. Secondly, modern quantitative computational ab initio methods can be synergetically combined with various experimen tal techniques thus enabling precise numerical characterization of molecular properties better than ever anticipated earlier. However, the role of theory is not exhausted in disclosing the fundamental laws of Nature and production of ever increasing sets of data of high accuracy. It has to provide additionally a means of systematization, recognition of regularities, and ratio nalization of the myriads of established facts avoiding in this way complete chaos. Additional problems are represented by molecular wavefunctions provided by the modern high-level computational quantum chemistry methods. They involve, in principle, all the information on molecular system, but they are so immensely complex that can not be immediately understood in simple and physically meaningful terms. Both of these aspects, categorization and interpretation, call for conceptual models which should be preferably pictorial, transparent, intuitively appealing and well-founded, being sometimes useful for semi quantitative purposes. They should reduce a posteriori the over whelming information embodied in intricate wavefunctions to manageable and memorable qualitative results and concepts. Physically sound models have to provide an interpretation of experimental and accurate ab initio results and serve as a tool in unearthing common roots between seemingly unrelated data. It is noteworthy that chemistry operating on the molecular level uses its own scales provided by characteristic, standard or gauge molecules. Hence, an adequate model should offer a rationale VIII Series Preface for the trends of changes of properties within families of related compounds. Last but not least, qualitative models should try to bridge a gap between rigorous quantum mechanics and the empirical concepts of the phenomenological chemistry, or to provide their reconciliation if possible. It follows that models, both quantitative and qualitative, are inevitable ingredients of the scientific method representing a link in the cognitive chain between theory and experiment. A harmonious development of molecular sciences requires a rather uniform progress in experimental research and quantum, com putational and interpretive, chemistry. For that purpose it is of utmost importance to establish a common language - a sort of scientific Esperanto - thus avoiding a scientific tower of Babel. Important words in the scientific vocabulary are qualitative concepts. It is the aim of the present series of books "Theoretical Models of Chemical Bonding" to provide a conceptual basis of modern interpretive quantum chemistry. It will be shown that there are qualitative models which meet all the attributes mentioned above. A general theme of the series is the most important building block of the macroworld - a molecule and its properties. The problems addressed cover a wide spectrum of topics ranging from the basic postulates of classical chemistry to a fine description of subtle spectroscopic properties, electron correlation and relativistic effects, intramolecular and intermolecular interactions, chemical reactivity and biochemical activity etc. Chapters are produced by a number of leading experts in the fields and they reflect much of the current thinking. No attempt has been made to avoid an overlap between the related articles, because some overlapping is necessary if continuity and coherent presentation of the topics considered is desired. A general level of theory is intermediate and excessive mathematical formalism is avoided wherever per mitted. In keeping with the general philosophy of the Series an emphasis has been laid on chemical phenomena and experimental facts and their interpretation in terms of qualitative models close to chemical intution. The simplest possible description of mole cular properties is striven for, carefully avoiding the Scylla of com plexity and the Charybdis of oversimplification. It was impossible to give a full account of quantitative models like the modern VB approach and high-performance MO procedures because of space limitations. Fortunately, they are well covered in other books and review articles. The introductory book of the Series is devoted to the notion of atoms immersed in chemical environments and to the concept of the molecular structure. A necessity for modelling in chemistry and its striking features are expounded in the Prologue. In the second book, which has been prepared in parallel, the third most Series Preface IX important postulate of classical chemistry is tackled: the pheno menon of covalent bonding and a wide variety of its manifestations. It is common wisdom today that molecular structure is the central and most fruitful theme of modern chemistry, biology, and to some extent of medicine. Knowledge of molecular geome tric structural parameters gives important clues for understanding electronic behaviour in molecules and it is a starting point for more detailed studies. It is, therefore, rather disappointing that it is extremely difficult, if not impossible, to derive the concept of molecular structure from the first principles of quantum mechanics (Chapter 1). It indicates at the same time that a deduc tive method is not always the best way for acquiring scientific information. Chapter 2 written by the late Professor O. E. Polansky shows that some molecular properties are influenced by topo logy alone. A brief discussion of the graph theoretical indices is given in the Prologue too. Two subsequent Chapters deal with molecular symmetry which, revealing some of the most fundamental laws of Nature, gives a penetrating insight into the structure and properties of matter. Interestingly enough, there is a kinship between molecules and elementary particles. Symmetry has a prominent role in chemical and biochemical reactions to mention only chirality for example. The following two chapters describe how good theory blended with experiment can provide valuable information about mole cular architecture. It illustrates rather nicely how the approxi mate but shrewed guesswork of Born, Oppenheimer and Huang yielded a model of molecular structure which proved enormously useful in rationalizing spatial arrangements of atoms and chemical bonds in nearly rigid molecules. It should be strongly pointed out in this connection that clamped nuclei approximation actually makes quantum mechanics a suitable theory for chemistry. Chapter 6 dwells on the simple ball-and-elastic spring model of molecules which underlies the molecular mechanics methods. Various applications to stereo-chemistry are thoroughly discussed. The last two chapters elaborate on another pivotal topic: a concept of atoms embedded in intramolecular potentials. It appears, namely, that molecules have a "good memory" and "remember" atoms which took part in their formation. In other words, atoms are not scrambled by chemical bonding and retain some individuality within molecular systems. Hence, the modified atoms are natural constituents and basic units of molecules, which is by no means obvious at first glance starting from the Schr6din ger equation. Bader's elegant "zero-flux" theory of modified atoms is discussed in Chapter 7. It turns out that a number of empirical notions of classical chemistry can be translated into quantum mechanical language by using the rigorously defined concept of topological atoms. The concluding chapter describes x Series Preface modified atoms in molecules at the qualitative model level illustrat ing a power and limitations of the conceptual approach. Naturally, the last two chapters could have been placed at the very beginning of this volume, for it is impossible to discuss molecular structure without adopting the atomic hypothesis. Hoewever, they both lean heavily on the use of the clamped nuclei approximation and the molecular geometry concept. Therefore, the latter had to be discussed first. The present Series will hopefully strengthen interplay and in crease creative interaction between the rigorous quantum theory, conceptual modelling of the electronic structure of molecules, and chemical phenomenology. Rapid concerted progress could be expected by increasing the overlap between these areas, because a fine balance between the very detailed results and development of unifying ideas is its conditio sine qua non. If this goal is at least partially fulfilled, our efforts will be greatly rewarded. It is also hoped that the interested reader will share some of the fascination and enthusiasm which played a great part in the pre paration of these volumes. Finally, I would like to express my sincere thanks to all the au thors for their fine contributions and support which made this endevour possible. Special thanks go to Professors E. Clementi, T. Cvitas, R. Gleiter, W. Kutzelnigg, W. H. E. Schwarz, J. To masi and M. C. Zerner for fruitful discussions and useful sugges tions. A good deal of editing was done during my multiple stays at the Organic Chemistry Institute of the University of Heidelberg and financial support from the Alexander von Humboldt-Foun dation is greatefully acknowledged. Z. B. Maksic Table of Contents Prologue: Modelling - A Search for Simplicity z. B. Maksic ..................... XIII The Concept of Molecular Structure B. T. Sutcliffe . . . . . . . . . Topology and Properties of Molecules O. E. Polansky . . . . . . . . . . 29 Symmetry in Molecules J. P. Dahl ..... . 75 Chirality of Molecular Structures - Basic Principles and Their Consequences L. D. Barron. . . . . . . . . . . . . . . . . . 115 Interplay of Experiment and Theory in Determining Molecular Geometries. A. The Experiments J. E. Boggs. . . . . . . . . . . . . . . . . . . . . . 153 Interplay of Experiment and Theory in Determining Molecular Geometries. B. Theoretical Methods J. E. Boggs. . . . . . . . . . . . . . . . . . . .. 185 Molecular Mechanics alias Mass Points and Elastic Springs Model of Molecules A. Y. Meyer . . . . . . . . . . . . . . . . . . . . . 213 Atoms in Molecular Evironments K. B. Wiberg. . . . . . . . . . . . . . . . . . . . . 255 The Modelling of Molecules as Collections of Modified Atoms Z. B. Maksic. . . . . . . . . . . . . . . . . . . . . 283 Prologue If you know a thing, it is simple, If it is not simple, you don't know it. Oriental proverb Modelling - A Search for Simplicity z. B. Maksic Ruder Boskovic Institute, POB 1016, 41001 Zagreb, Croatia, Yugoslavia and Faculty of Natural Sciences and Mathematics, University of Zagreb, Marulicev trg 19, 41000 Zagreb, Croatia, Yugoslavia 1 Introduction Modelling is an important ingredient of the scientific method. It is present in all sciences including humanities as an indispensable tool for tackling otherwise intractable complex problems and phenomena. We shall briefly comment here on modelling in chemistry with particular emphasis on theoretical models. It should be noted in passing that the present reflections are general and that they might be of some relevance in other natural sciences too. There is no precise and unique definition of a model and con sequently different people have different opinion about its notion, meaning and relevance, which is sometimes a cause for serious misconceptions and heated disussion. The reason is that models vary considerably in their nature and serve quite different pur poses. Another reason is that the art of modelling is not always mastered leading to artificial results and consequently to a low reputation of models in general. To put it succinetly, the defi nition, classification and taxonomy of models is subjective and perhaps they should be modelled themselves. Hence, what follows is necessarily a personal view. The interested reader may also consult several scholarly written reviews [1-6] and books [7-9] or special subject issues of some journals [10-12]. Needless to say, he is also encouraged to read the contributions collected in this series of book written by experts in the field, who discuss
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