Rolf Gleiter, Gebhard Haberhauer

Aromaticity and Other Conjugation EffectsWith a Foreword by Roald Hoffmann

Foreword

I still remember the day when I first looked at the large accordion-folded sheets that a noisydot-matrix spewed forth, sheets that contained the output of an extendedH!uckel calculationon pyridazine, pyrimidine, and pyrazine. The wave functions emerged as a 28! 28matrix,running across several pages; one had to scan down a long column to assign symmetries tothe 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 lesslocalized at N. Every chemist expected to see them; I was a chemist. I already knew from apyridine calculation that the lone pairs weren�t as localized as I thought they would be. Inthe diazine series I expected to see the lone pairs split by a lot in energy in pyridazine, due tolone-pair–lone-pair overlap, less so in pyrimidine, still less in pyrazine. No different fromanyone 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 betweenthe 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 fromthe 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 couldn�t even get that result into my 1964 paper on s orbitals in azines; friends didn�t

believe it. And while the result perplexed me, it took the slow building of confidence inorbital interaction diagrams, and my own rediscovery of qualitative perturbation theory(both clearly arising from the fertile interaction withR. B.Woodward) before I could hazarda simple, symmetry- and overlap-based explanation for that pyrazinefinding. Youwillfind itin a chapter in this book.

The purpose of telling this story is not solipsistic. What you see in this wonderful book istheory and experiment at the nexus of maximal understanding. It is where the authors ofthis book and I were fortunate enough to be in our scientific journey. And it is wherecontemporary quantum chemistry is not. Yet it is a place of the intellect to where I amconfident electronic structure theory in chemistry must eventually wend its way.

We are in the age of simulation. Bigger and better calculations, winding their waythrough a maze of functionals, basis sets, and ways of including correlation, give anobservable to almost any desired degree of accuracy.With little explanation –none from the

jVII

computer, of course, little from the man or woman guiding the computer. If you want thesame observable for a slightly modified molecule, a methyl replacing a hydrogen, theprescription is to go back into the computer. It�s as if there were an uncertainty principlerelating chemical understanding and computational accuracy. There are exceptions, but byand large the complexity of what one has to do to get, say, an electronic spectrum, right, andthe psychology of human-machine interactions militate against accepting simple, quali-tative explanations. Such as the ones you will find in this book.Meanwhile, our graduate students andundergraduates desperately craveunderstanding,

not numbers. Actually, so do their teachers. So what we teach is just that simple orbitalargument, the perturbation-theory-based mechanics of interacting orbitals. The studentslap it up, for the desire to understand is so strong!The disjunction that these students, now become professionals, face when they turn to

today�s computational theoretical chemist for assistance in their research, may be trau-matic. People don�t write about this moment of collision of simple (true, often too simple),learned explanations with calculations; there is no place for describing that emotionalexperience in our papers. But I see the traces, papered over, as I read the literature. And Ismile 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 bestelectronic structure computations of the day; (b) the experimental tools by which one canprobe interaction, foremost among themphotoelectron and electronic spectroscopy; and (c)qualitative (yet quantifiable) molecular-orbital-based reasoning. In doing so, this book�srehearsal of a journey of understanding that goes back decades is also a signpost for thefuture. 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 youngpeoplewill, one day, use their computers� rich harvest and infinite potentialities for probingalternative realities intelligently, as a numerical laboratory. Not just for rationalizing orpredicting an observable, but for building chemical understanding. I am confident that thepath to understanding set forth in this volume will survive in that future.

Roald Hoffmann

VIIIj Foreword

Aromaticity andOther Conjugation

EffectsThis is a superb book that

covers exactly what the titleclaims, and it does so in a thorough,

highly organized, and readable style.The first author, Professor Rolf Gleiter, at

the University of Heidelberg, has been amajor contributor to this field for more than 40years and is intimately familiar with both itshistorical development and its current state. Ineach chapter, the authors systematically reviewlandmark advances in the field, generallybeginning with key experimental observationsand then explaining them by simple molecularorbital arguments based on perturbation theorywithin a one-electron model, e.g., H!ckel MOtheory and qualitative orbital interactiondiagrams. The overall picture is then furtherrefined by the presentation of additionalexperimental data and results from ab initiocalculations.

The didactic, story-telling nature of the writingderives from Professor Gleiter"s many years ofteaching this subject to organic chemistry studentsand makes this an excellent, up-to-date text bookfor any professors who might like to teach a similarcourse. For professors who teach broader courses inphysical organic chemistry, the book can serve as agold mine of ready-made lecture notes on a widerange of special topics, with a full complement ofliterature references for each. All the key papersand virtually every review article and previousbook on the topics covered are included in thebibliography, which cites more than 2000 publica-tions. Professional chemists who have alreadycompleted their academic training but want tolearn more about aromaticity and other conjuga-tion effects will find no better source than this self-contained volume. Experts in the field shouldconsider this book a must for their personalcollections.

Every chemistry library should own it. Studentsof organic chemistry who are newcomers to thisfield and want to know more will find everythingthey need in this authoritative book. The paper-back version is affordable to students at less thanhalf the price of the hard cover book. An appendixwith character tables for selected symmetry groupsand twelve pages of index are helpful; inclusion ofan author index would have made the book evenbetter.

A familiarity with the principles of molecularorbital theory and the H!ckel MO method is aprerequisite for readers who want to get the mostout of this book. Expertise in quantum mechanics isnot essential, but those who have never taken a

course in the subject will find themselves disadvan-taged. A standard one-year undergraduate coursein organic chemistry is the only other backgroundthat the authors assume. The final chapter of thebook offers a 48-page review of the theoreticalmethods used throughout the book, starting fromthe Schrçdinger equation, and progressing throughthe LCAO-MO method, H!ckel and extendedH!ckel theory, ab initio Hartree–Fock procedures,semiempirical SCF methods, and the various waystheoreticians have dealt with the problem ofelectron correlation, including the popular densityfunctional approach. Readers seeking a refresherwill appreciate this section of the book, and expertscan skip it, but the uninitiated will find it roughsledding. The chapter ends with a clear discussionabout qualitative rules for the interactions oflocalized orbitals and a primer on spectroscopicmethods for detecting conjugation effects. Exper-imental results from UV/Vis spectroscopy andphotoelectron spectroscopy appear again andagain throughout the book, and there is no onebetter qualified than Professor Gleiter, the world"sforemost champion of photoelectron spectroscopy,to enlighten those who are unfamiliar with thispowerful technique for probing the molecularorbitals of molecules.

The main body of the book begins with simpleconjugated polyenes and polyynes, moves on tocyclic conjugation, and spends a long time discus-sing aromaticity and the various criteria that havebeen used to try to quantify it. The section on whybenzene adopts D6h instead of D3h symmetry is oneof the clearest explanations I have ever read.Chapter 1 ends with more than 50 pages onpolycyclic aromatic hydrocarbons, graphenesheets, bowl-shaped (geodesic) polyarenes andfullerenes, and, finally, with Heilbronner–Mçbiusrings and ribbons. As in all the chapters, thepresentation is rich in graphics, including manyspectra, data tables, and figures reproduced withpermission from the original literature. Theabsence of color graphics is unfortunate.

Abbreviated synthesis schemes are sprinkledhere and there, which many readers will appreciate,but the focus is firmly on the relationship betweenthe structure and the electronic properties ofmolecules.

Chapter 2, on through-space interactions, tracesthe roots of the homoconjugation concept to classicexperiments on the cholesteryl cation in the 1940sand describes how the notion of homoaromaticitysubsequently came to be recognized. The treatmentof transannular effects is not confined to hydro-carbon systems but also includes fundamentalstudies on interactions of non-bonding electronpairs on divalent sulfur and tertiary nitrogen atomswith transannular carbonyl groups. Detailed dis-cussions about “proton sponges” and spiroconju-

Aromaticity and OtherConjugation EffectsBy Rolf Gleiter and GebhardHaberhauer. Wiley-VCH,Weinheim, 2012. 452 pp.,hardcover, E 129.00.—ISBN978-3527329465 (softcover,E 59.00.—ISBN 978-3527329342)

.AngewandteBooks

AngewandteChemie

2403Angew. Chem. Int. Ed. 2013, 52, 2403 – 2404 ! 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

gation round out the chapter. The venerable subjectof through-space interactions in donor–acceptor(D–A) complexes between planar arenes is treatedseparately in Chapter 3. First observed as charge-transfer bands in UV/Vis absorption spectroscopy,other consequences of such D–A interactions showup in crystal packing motifs, and they have beenexploited with great success for the synthesis ofrotaxanes and catenanes. Chapter 3 ends with anexcursion into the realm of convex–concave inter-actions involving ball-, bowl-, and belt-shapedconjugated systems. Biochemists concerned withp-stacking, energy transfer, and electron transferbetween aromatic rings will find this chapterparticularly useful.

In many cases, through-bond interactions arelarger than through-space interactions, even overseveral bond lengths, on account of direct s-bonding, and these interactions are discussed inChapter 4. As in the earlier chapters, the scope ofthe treatment includes not only hydrocarbon p-systems but also interactions involving heteroatomnon-bonding electron pairs. The unique capabilityof certain rings and cages to electronically connectdistant electron pairs is especially intriguing.

Chapter 5 deals with hyperconjugation, animportant stereoelectronic effect that has beenused to explain various properties and reactivitiesof molecules since the 1930s. A distinction is madebetween positive and negative hyperconjugation,and each class is then divided into different typesdepending on which orbitals are involved in theinteraction. Natural Bond Orbital (NBO) analysesreveal trends in the hyperconjugative abilities ofvarious donor and acceptor orbitals.

By compiling decades of lecture notes andtransforming them into a published book, theauthors have performed a tremendous service tothe organic chemistry community. We concurenthusiastically with Professor Roald Hoffmann,who says in the Foreword, “This volume providestrue understanding, which is the best thing one cansay of any book.”

Lawrence T. Scott, Hee Yeon Cho, Maria N. Eliseeva,Edward A. Jackson, Takayuki Tanaka,Tomoharu TanikawaMerkert Chemistry Center, Department of ChemistryBoston College, Chestnut Hill, Massachusetts (USA)

DOI: 10.1002/anie.201209331

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2404 www.angewandte.org ! 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2013, 52, 2403 – 2404

Contents

Foreword VIIPreface XIII

1 Conjugated pp Systems 11.1 Linear Conjugated Polyenes 11.1.1 Ground State Properties of Conjugated Polyenes 11.1.2 Spectroscopic Properties of Conjugated Polyenes 61.1.3 Symmetric Cyanine Dyes and the FEM Model 101.1.4 Photoelectron Spectra of Conjugated Polyenes 131.1.5 Long Polyene Chains (Polyacetylene) 151.2 Conjugated Oligoalkynes 171.2.1 Ground State Properties of Conjugated Oligoalkynes 171.2.2 Electronic Absorption Spectra of Conjugated Oligoalkynes 201.2.3 Photoelectron Spectra of Conjugated Oligoalkynes 201.3 Conjugated Planar Monocyclic p Systems 221.3.1 Historical Remarks 221.3.2 MO Description of Annulenes – (4n)p and (4n!2)p Electron

Systems 241.4 Criteria for Aromaticity 281.4.1 Estimation of HMO Delocalization Energies 281.4.2 Dewar and Hess-Schaad Resonance Energies 301.4.3 Thermochemical Approach to Aromaticity 321.4.4 Isodesmic and Homodesmotic Reactions 341.4.5 Ring Current Effects 361.4.6 Bond Indices (HOMA) 451.5 Structures of Monocyclic (4n!2)p and (4n)p Annulenes 461.5.1 Experimental Data for Benzene 461.5.2 Why Does Benzene Adopt D6h and Not D3h Symmetry? 471.5.3 Structures of Higher (4n!2)p Annulenes 531.5.4 Structures of (4n)p Annulenes – Less Symmetry More Stability 561.6 Conjugated Polycyclic Planar p Electron Systems 621.6.1 Polycyclic Aromatic p Systems 641.7 Substituent Effects 75

jIX

1.7.1 Linear Free Energy Relations 761.7.2 Effects of Substituents on the Excited States of Aromatic

Molecules 791.8 Conjugation in Two and Three Dimensions 821.8.1 Conjugated PAH Sheets and Graphene 831.8.2 Bowl-Shaped Polyarenes and Fullerenes 871.8.3 Hoop-Shaped p Electron Systems 1011.8.4 Heilbronner-Möbius Rings and Ribbons 110

References 122

2 Through-Space Interactions between pp Systems 1332.1 Homoconjugation 1332.2 Transannular Effects 1352.2.1 Double Bonds and Carbenium Ions 1352.2.2 tert-Nitrogen Centers and Carbonyls 1372.2.3 Transannular Interaction of Two Nitrogen Centers 1422.2.4 Divalent Sulfur Centers and Carbonyls 1482.2.5 Transannular Hydride Shifts 1502.2.6 Long Distance Metal!p" Interactions 1542.3 Homoaromatic Systems 1562.3.1 Homoaromatic Cations 1572.3.2 Homoaromaticity of Neutral Systems 1592.4 Spiroconjugation 1662.4.1 Basic Concepts 1662.4.2 Spectroscopic Evidence for Spiroconjugation 171

References 176

3 Donor–Acceptor Interactions between Planar Arenes 1813.1 Donor-Acceptor Complexes 1813.2 Structures of Benzene and Related Aromatics in the

Solid State 1873.3 Interactions between Molecules of Opposite Electric Quadrupole

Moments 1893.4 Model Studies to Measure the Strength of Non-Covalent

Interactions Between p Systems in Organic Solvents 1903.5 Model Calculations on p–p Interactions 1963.5.1 p–Electron Point Charge Model 1963.5.2 Ab Initio Calculations 1993.6 Applications and Consequences of p–p Interactions of Arenes in

Chemistry 2013.6.1 Preorganized Hosts for p–p Complexation 2033.6.2 Rotaxanes and Catenanes 2053.6.3 Concave–Convex Interactions in Ball-, Bowl- and Belt-Shaped

Conjugated Systems 209References 213

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4 Through-Bond Interactions between pp Systems and Non-Bonded Electron Pairsof Heteroatoms 217

4.1 Theoretical Models 2174.2 Dehydroaromatics 2234.3 Through-Bond Interactions between Non-Conjugated

p Systems 2264.3.1 Through-Bond Interactions between Olefinic Groups 2264.3.2 Through-Bond Interactions between Non-Conjugated

Aromatic Units 2354.3.3 Through-Bond Interactions between Triple Bonds 2384.3.4 Through-Bond Interactions via Rings and Cages 2424.3.5 Through-Bond Interactions between Non-Bonding Electron

Pairs of Heteroatoms 2554.3.6 Through-Bond Coupling between Chromophores 2624.4 Rationalization of Intramolecular Reactivity by

Through-Bond Coupling 2654.4.1 Grob Fragmentation 2654.4.2 Regiochemistry of the Intramolecular [2!2] Photocycloaddition 272

References 279

5 Hyperconjugative Interactions 2835.1 Concept of the Two-Electron/Two-Orbital Interactions 2835.2 Definition and Manifestation of Ground State Properties 2885.2.1 Definition and Trends in Hyperconjugative Donor and Acceptor Abilities 2885.2.2 Influence of Hyperconjugation on Ground State Properties 2945.3 Positive Hyperconjugation (s"p, s"p# and s"s# Interactions) 2965.3.1 C–H and C–C Bonds of Alkyl Groups as Donors in s"p and s"p#

Hyperconjugative Interactions 2965.3.2 C–C Bonds of Cyclopropyl as Donors in s"p and s"p# Hyperconjugative

Interactions 3055.3.3 C–H and C–C Bonds of Alkyl Groups as Donors in s"s# Hyperconjugative

Interactions 3115.3.4 C–M Bonds as Donors in Hyperconjugative Interactions 3225.4 Negative Hyperconjugation (n"s# and p"s# Interactions) 3295.4.1 The Anomeric Effect – nO and nN as Donors in n"s# Hyperconjugative

Interactions 3295.4.2 nF and nCl as Donors in n"s# Hyperconjugative Interactions 3425.4.3 nC(") as Donors in n"s# Hyperconjugative Interactions 3445.4.4 p"s# Hyperconjugative Interactions 353

References 356

6 Theoretical Models 3616.1 Quantum Chemical Calculation Methods – An Overview 3616.1.1 Schrödinger Equation 3616.1.2 The Variational Theorem 363

Contents jXI

6.1.3 The Orbital Approximation 3656.1.4 Molecular Orbitals – The LCAO-MO Method 3676.1.5 Hückel Molecular Orbital Theory (HMO Theory) 3746.1.6 Extended Hückel Theory 3826.1.7 Ab initio Hartree-Fock Procedures 3836.1.8 Semi-Empirical SCF Methods 3896.1.9 Procedures for Taking the Correlation Energy into Account 3906.2 Orbital Interactions 3966.2.1 Qualitative Rules for the Interactions of Localized Orbitals 3966.2.2 The LCBO Theory and Group Orbitals 4006.2.3 Orbitals of Cyclopropane and Cyclobutane 4036.2.4 Localized and Delocalized Orbitals 4066.3 Spectroscopic Methods for Detecting Conjugation Effects 4096.3.1 Photoelectron Spectroscopy 4096.3.2 UV-Vis Spectroscopy 415

References 426

7 Appendix 4297.1 Character Tables for Selected Symmetry Groups 4297.2 Basic Equations for Nuclear Magnetic Shielding in Molecules 436

Reference 4387.3 Energy Conversion Table and Abbreviations 438

Subject Index 441

XIIj Contents