Aromaticity and Other Conjugation Effects

Rolf Gleiter, Gebhard Haberhauer Aromaticity and Other Conjugation Effects With a Foreword by Roald Hoffmann VII j Foreword I still remember the day when I first looked at the large accordion-folded sheets that a noisy dot-matrix spewed forth, sheets that contained the output of an extended Huckel€ calculation on pyridazine, pyrimidine, and pyrazine. The wave functions emerged as a 28 28 matrix, Â running across several pages; one had to scan down a long column to assign symmetries to the levels and see where in the molecules the largest coefficients were. In the MOs of these diazines I was naturally looking for two lone pairs more or less localized at N. Every chemist expected to see them; I was a chemist. I already knew from a pyridine calculation that the lone pairs werent as localized as I thought they would be. In the diazine series I expected to see the lone pairs split by a lot in energy in pyridazine, due to lone-pair–lone-pair overlap, less so in pyrimidine, still less in pyrazine. No different from anyone else, I was thinking the problem through in a through-space direct overlap way. Pyridazine followed expectations. Pyrimidine was a bit of a surprise – the gap between the lone pairs was larger than I expected. But the real shock was pyrazine – the 1,4-diazine – the gap between the two mainly-N-localized orbitals was not tiny, but a few eV. And the wrong combination, the antisymmetric one, was lower in energy! Wrong because from the perspective of through-space interactions I expected the symmetric combination lower – not that I would have expected any significant interaction between two orbitals 2.8 A apart, and pointing away from each other. I couldnt even get that result into my 1964 paper on s orbitals in azines; friends didnt believe it. And while the result perplexed me, it took the slow building of confidence in orbital interaction diagrams, and my own rediscovery of qualitative perturbation theory (both clearly arising from the fertile interaction with R. B. Woodward) before I could hazard a simple, symmetry- and overlap-based explanation for that pyrazine finding. You will find it in a chapter in this book. The purpose of telling this story is not solipsistic. What you see in this wonderful book is theory and experiment at the nexus of maximal understanding. It is where the authors of this book and I were fortunate enough to be in our scientific journey. And it is where contemporary quantum chemistry is not. Yet it is a place of the intellect to where I am confident electronic structure theory in chemistry must eventually wend its way. We are in the age of simulation. Bigger and better calculations, winding their way through a maze of functionals, basis sets, and ways of including correlation, give an observable to almost any desired degree of accuracy. With little explanation – none from the VIII Foreword j computer, of course, little from the man or woman guiding the computer. If you want the same observable for a slightly modified molecule, a methyl replacing a hydrogen, the prescription is to go back into the computer. Its as if there were an uncertainty principle relating chemical understanding and computational accuracy. There are exceptions, but by and large the complexity of what one has to do to get, say, an electronic spectrum, right, and the psychology of human-machine interactions militate against accepting simple, qualitative explanations. Such as the ones you will find in this book. Meanwhile, our graduate students and undergraduates desperately crave understanding, not numbers. Actually, so do their teachers. So what we teach is just that simple orbital argument, the perturbation-theory-based mechanics of interacting orbitals. The students lap it up, for the desire to understand is so strong! The disjunction that these students, now become professionals, face when they turn to todays computational theoretical chemist for assistance in their research, may be trau- matic. People dont write about this moment of collision of simple (true, often too simple), learned explanations with calculations; there is no place for describing that emotional experience in our papers. But I see the traces, papered over, as I read the literature. And I smile as I see them. This volume provides true understanding, which is the best thing one can say of any book. The book reaches at every point for a conciliation of three threads: (a) the best electronic structure computations of the day; (b) the experimental tools by which one can probe interaction, foremost among them photoelectron and electronic spectroscopy; and (c) qualitative (yet quantifiable) molecular-orbital-based reasoning. In doing so, this books rehearsal of a journey of understanding that goes back decades is also a signpost for the future. For there will surely come a time, and there will come theoreticians for that time, who will do state-of-the-art calculations not just for the numbers. Instead bright young people will, one day, use their computers rich harvest and infinite potentialities for probing alternative realities intelligently, as a numerical laboratory. Not just for rationalizing or predicting an observable, but for building chemical understanding. I am confident that the path to understanding set forth in this volume will survive in that future. Roald Hoffmann Angewandte. Angewandte Books Chemie Aromaticity and course in the subject will find themselves disadvan- Other Conjugation taged. A standard one-year undergraduate course Effects in organic chemistry is the only other background that the authors assume. The final chapter of the This is a superb book that book offers a 48-page review of the theoretical covers exactly what the title methods used throughout the book, starting from claims, and it does so in a thorough, the Schrçdinger equation, and progressing through highly organized, and readable style. the LCAO-MO method, Hückel and extended The first author, Professor Rolf Gleiter, at Hückel theory, ab initio Hartree–Fock procedures, the University of Heidelberg, has been a semiempirical SCF methods, and the various ways major contributor to this field for more than 40 theoreticians have dealt with the problem of years and is intimately familiar with both its electron correlation, including the popular density historical development and its current state. In functional approach. Readers seeking a refresher each chapter, the authors systematically review will appreciate this section of the book, and experts landmark advances in the field, generally can skip it, but the uninitiated will find it rough beginning with key experimental observations sledding. The chapter ends with a clear discussion and then explaining them by simple molecular about qualitative rules for the interactions of orbital arguments based on perturbation theory localized orbitals and a primer on spectroscopic within a one-electron model, e.g., Hückel MO methods for detecting conjugation effects. Exper- theory and qualitative orbital interaction imental results from UV/Vis spectroscopy and diagrams. The overall picture is then further photoelectron spectroscopy appear again and refined by the presentation of additional again throughout the book, and there is no one experimental data and results from ab initio better qualified than Professor Gleiter, the worlds calculations. foremost champion of photoelectron spectroscopy, The didactic, story-telling nature of the writing to enlighten those who are unfamiliar with this derives from Professor Gleiters many years of powerful technique for probing the molecular teaching this subject to organic chemistry students orbitals of molecules. and makes this an excellent, up-to-date text book The main body of the book begins with simple for any professors who might like to teach a similar conjugated polyenes and polyynes, moves on to course. For professors who teach broader courses in cyclic conjugation, and spends a long time discus- physical organic chemistry, the book can serve as a sing aromaticity and the various criteria that have gold mine of ready-made lecture notes on a wide been used to try to quantify it. The section on why range of special topics, with a full complement of benzene adopts D6h instead of D3h symmetry is one literature references for each. All the key papers of the clearest explanations I have ever read. and virtually every review article and previous Chapter 1 ends with more than 50 pages on book on the topics covered are included in the polycyclic aromatic hydrocarbons, graphene bibliography, which cites more than 2000 publica- sheets, bowl-shaped (geodesic) polyarenes and tions. Professional chemists who have already fullerenes, and, finally, with Heilbronner–Mçbius completed their academic training but want to rings and ribbons. As in all the chapters, the learn more about aromaticity and other conjuga- presentation is rich in graphics, including many tion effects will find no better source than this self- spectra, data tables, and figures reproduced with contained volume. Experts in the field should permission from the original literature. The consider this book a must for their personal absence of color graphics is unfortunate. collections. Abbreviated synthesis schemes are sprinkled Every chemistry library should own it. Students here and there, which many readers will appreciate, of organic chemistry who are newcomers to this but the focus is firmly on the relationship between field and want to know more will find everything the structure and the electronic properties of they need in this authoritative book. The paper- molecules. back version is affordable to students at less than Chapter 2, on through-space interactions, traces half the price of the hard cover book. An appendix the roots of the homoconjugation concept to classic with character tables for selected symmetry groups experiments on the cholesteryl cation in the 1940s and twelve pages of index are helpful; inclusion of and describes how the notion of homoaromaticity an author index would have made the book even subsequently came to be recognized.

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