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Introduction t0

SPECTROSCOPY third edition

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THIRD EDITION INTRODUCTION TO SPECTROSCOPY A GUIDE FOR STUDENTS OF ORGANIC CHEMISTRY Donald L Pavia Gary M Lampman George S Kriz Department of Chemistry Western Washington University Bellingham, Washington BROOKS/COLE THOMSON LEARNING

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BROOKS/COLE

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THOMSON LEARNING

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PREFACE

his is the third edition of our textbook in spectroscopy intended for students of organic chemistry This textbook can serve as a supplement for the typical organic chemistry lecture

textbook, and it can also be used as a “stand-alone” textbook for an advanced undergraduate

course in spectroscopic methods of structure determination This book is also a useful tool for stu- dents engaged in research Our aim is not only to teach students to interpret spectra, but also to pre- sent basic theoretical concepts As with the previous editions, we have tried to focus on the important aspects of each spectroscopic technique without dwelling excessively on theory or com- plex mathematical analyses

This book is a continuing evolution of materials that we use in our own courses, both as a sup- plement to our organic chemistry lecture course series and also as the principal textbook in our upper division courses in spectroscopic methods and advanced NMR techniques Explanations and examples that we have found to be effective in our courses as well as in our reviewers’ courses have been incorporated into this edition

New to This Edition

This third edition of Introduction to Spectroscopy contains some important changes We have ex- panded our earlier chapter on advanced considerations in nuclear magnetic resonance spectroscopy into two new chapters The first of these chapters (Chapter 5) focuses on spin coupling interactions, while the second new chapter (Chapter 6) discusses protons on heteroatoms, exchange phenomena, tautomerism, valence tautomerism, spin decoupling methods, and chemical shift reagents Throughout the book we have also increased the number of 300-MHz spectra in order to provide spectra that compare more closely with those that students might obtain using modern instrumenta- tion We have provided expansions for many spectra so that students can see more clearly the multi- plicity of the peaks Hertz values have been included on the expansions to facilitate the determination of coupling constants A section has been added in Chapter 5 that shows students how to extract the coupling constants in complex allylic systems

We have included a discussion of some new techniques including NOE difference spectroscopy We have also expanded our discussion of the information available from a DEPT experiment We have added more tables of spectral data, and we have expanded the appendices to include more ta- bles of chemical shift and coupling constant values

We have added Spectral Analysis Boxes to the chapters on nuclear magnetic resonance and mass spectrometry In the previous edition, these boxes were a feature used only in the chapter on infrared

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Preface

spectroscopy We hope that these summary boxes will help students understand better how to use the information that the spectra contain In addition, these boxes are part of a survey of NMR and mass spectral data by functional group, a new feature of this edition

We have also added many new problems to each chapter This book thus contains a great many _ problems, with a wide range of difficulty, ranging from relatively simple to more challenging Stu- dents are provided answers to some of the problems, however, at the request of reviewers and in- structors using the book, we have included some problems that are not answered at the back of the book To make assignments easier for instructors, we have indicated the problems that have student answers with an asterisk (*) Answers to problems without student answers will be made available to qualified instructors via the World Wide Web Chapter 9 (Combined Structure Problems) contains many new and challenging examples, most of them with 300-MHz spectra that include expansions

Problems making use of two-dimensional NMR data have been added to Chapter 10 Acknowledgments

We want to express our deep gratitude to colleagues who have contributed significantly to this new edition Our colleague, Prof James R Vyvyan (Western Washington University) made many valu- able suggestions and provided several examples from his research Charles Wandler, the instru- ments technician at Western Washington University did yeoman duty running spectra for us (running is his specialty!) Meagan Lindstrom, a chemistry undergraduate student, also ran many spectra Special thanks are due to the students in our spectroscopy classes for the stimulation they provide us

We wish to acknowledge the cooperation of Varian Associates and the Aldrich Chemical Company for their permission to use infrared and nuclear magnetic resonance spectra from their catalogues

Many of our colleagues reviewed our manuscript and provided helpful criticism: Professors Jonathan Touster (Stanford University), Charles Garner (Baylor University), William F Wood (Hum- boldt State University), and Stephen Branz (San Jose State University) Saunders College Publishing and TSI Graphics handled production of this textbook Sandi Kiselica, our Developmental Editor, worked very hard keeping us up to the production schedule and providing us with a great deal of help Tom Robinson at TSI Graphics capably handled the final production stages of the book

Finally, once again we must thank our wives, Neva-Jean, Marian, and Carolyn, for the support and their patience They deserve to have a break from dealing with piles of manuscript pages and Spectra scattered over their dining room tables!

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CHAPTER 1

MOLECULAR FORMULAS AND WHAT CAN BE LEARNED

FROM THEM 1

11 Elemental Analysis and Calculations 1 1.2 Determination of Molecular Mass 3 1.3 Molecular Formulas 4

14 Index of Hydrogen Deficiency 4 1.5 The Rule of Thirteen 7

1.6 A Quick Look Ahead to Simple Uses of Mass Spectra 10 Problems 11

References 12

CHAPTER 2

INFRARED SPECTROSCOPY 13

2.1 The Infrared Absorption Process 14 2.2 _ Uses of the Infrared Spectrum 15

2.3 The Modes of Stretching and Bending 16

2.4 Bond Properties and Absorption Trends 18 2.5 The Infrared Spectrometer 20

A Dispersive Infrared Spectrometers 20 B Fourier Transform Spectrometers 23

2.6 Preparation of Samples for Infrared Spectroscopy 23 2.7 What To Look for When Examining Infrared Spectra 24 2.8 Correlation Charts and Tables 27

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viii Contents 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 Hydrocarbons: Alkanes, Alkenes, and Alkynes 29 A Alkanes 29 B Alkenes 31 C Alkynes 33 Aromatic Rings 41 Alcohols and Phenols 45 Ethers 48 Carbonyl Compounds 50 A Factors That Influence the C=O Stretching Vibration 52 B Aldehydes 54 C Ketones 56 D Carboxylic Acids 60 E, Esters 62 F Amides 68 G Acid Chlorides 70 H Anhydrides 71 Amines 72 Nitriles, Isocyanates, Isothiocyanates, and Imines 75 Nitro Compounds 77 Carboxylate Salts, Amine Salts, and Amino Acids 78 Sulfur Compounds 79 Phosphorus Compounds 82 Alkyl and Aryl Halides 82 Problems 84 References 101 CHAPTER 3

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

PART ONE: BASIC CONCEPTS 102 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10

Nuclear Spin States 102 Nuclear Magnetic Moments 103

Absorption of Energy 104

The Mechanism of Absorption (Resonance) 106 Population Densities of Nuclear Spin States 108 The Chemical Shift and Shielding 109

The Nuclear Magnetic Resonance Spectrometer 111 A The Continuous-Wave (CW) Instrument 111 B The Pulsed Fourier Transform (FT) Instrument 113 Chemical Equivalence—A Brief Overview 117 Integrals and Integration 118

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Contents ix 3.11 Local Diamagnetic Shielding 121 A Electronegativity Effects 121 B Hybridization Effects 123 C Acidic and Exchangeable Protons: Hydrogen Bonding 124 3.12 Magnetic Anisotropy 125

3.13 Spin—Spin Splitting (7 + 1) Rule 128 3.14 The Origin of Spin-Spin Splitting 131 3.15 The Ethyl Group (CHaCHạ—) 133 3.16 Pascal's Triangle 134

3.17 The Coupling Constant 134

3.18 A Comparison of NMR Spectra at Low and High Field Strengths 137 3.19 Survey of Typical 'H NMR Absorptions by Type of Compound 138 A Alkanes 139 B Alkenes 140 C Aromatic Compounds 140 D Alkynes 142 E Alkyl Halides 143 F Alcohols 144 G Ethers 145 H Amines 146 I Nitriles 147 J Aldehydes 148 K Ketones 149 L Esters 150 M Carboxylic Acids 150 N Amides 152 O Nitroalkanes 153 Problems 154 References 166 CHAPTER 4

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

PART TWO: CARBON-13 SPECTRA, INCLUDING HETERONUCLEAR COUPLING WITH OTHER NUCLEI 167

41 The Carbon-13 Nucleus 167 4.2 Carbon-13 Chemical Shifts 168

A Correlation Charts 168

B Calculation of '*C Chemical Shifts 170

4.3 Proton-Coupled °C Spectra—Spin—Spin Splitting of Carbon-13 Signals 171 44 ProtonDecoupledlC§pecra 173

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Contents 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15

Cross-Polarization: Origin of the Nuclear Overhauser Effect 176

Problems with Integration in °C Spectra 179

Molecular Relaxation Processes 180 Off-Resonance Decoupling 182 A Quick Dip into DEPT 182

Some Sample Spectra—Equivalent Carbons 185 Compounds with Aromatic Rings 187

Carbon-13 NMR Solvents—Heteronuclear Coupling of Carbon to Deuterium 189 Heteronuclear Coupling of Carbon to Fluorine-19 193

Heteronuclear Coupling of Carbon to Phosphorus-31 194 Problems 195

References 216

CHAPTER 5

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

PART THREE: SPIN-SPIN COUPLING 217 5.1 5.2 5.3 5.4 5.5 5.6 5.7 3.8 5.9 5.10

Coupling Constants: Symbols 217

Coupling Constants: The Mechanism of Coupling 218 A One-Bond Couplings (17) — 219 B Two-Bond Couplings (77) 220 C Three-Bond Couplings (7/) 223 D Long-Range Couplings (*J-"J) 227 Magnetic Equivalence 228 Nonequivalence Within a Group—The Use of Tree Diagrams when the n+ 1 Rule Fails 231

Measuring Coupling Constants from First Order Spectra 234 A Simple Multiplets—One Value of J (One Coupling) 234 B More Complex Multiplets—More Than One Value of J 235 Alkenes 237

Mechanisms of Coupling in Alkenes; Allylic Coupling 238

Measuring Coupling Constants—Analysis of an Allylic System 241 Is the ø + 1 Rule Ever Really Obeyed? 246

Second-Order Spectra—Strong Coupling 247 A First-Order and Second-Order Spectra 247 B Spin System Notation 248

C The A», AB, and AX Spin Systems 248

D The AB; AX¿ and AzB; A;X; Spin Systems 251 E Simulation of Spectra 254

F The Absence of Second-Order Effects at Higher Field 254 G Deceptively Simple Spectra 255

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5.11 Aromatic Compounds—Substituted Benzene Rings 255 A Monosubstituted Rings 256 B para-Disubstituted Rings 259 C Other Substitution 261 5.12 Coupling in Heteroaromatic Systems 264 5.13 Long-Range Coupling 265

5.14 Homotopic, Enantiotopic, and Diastereotopic Systems 268

5.15 Spectra of Diastereotopic Systems 270

Contents xi

A Diastereotopic Methyl Groups: (S)-(+)-3-Methyi-2-butanol 270 B Diastereotopic Hydrogens: 1,2-Dichloropropane 273

C Diastereotopic Fluorines: 1-Bromo-2-chloro-1,1,2-trifluoroethane 273 Problems 275

References 304

CHAPTER 6

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

PART FOUR: OTHER TOPICS IN ONE-DIMENSIONAL NMR 306

6.1 Protons on Oxygen: Alcohols 306 6.2 Exchange in Water and D,O 309

A Acid/Water and Alcohol/Water Mixtures 309 B Deuterium Exchange 310

C Peak Broadening Due to Exchange 311 6.3 Other Types of Exchange: Tautomerism 312 6.4 Protons on Nitrogen: Amines 314

6.5 Protons on Nitrogen: Quadrupole Broadening and Decoupling 6.6 Amides 319

6.7 The Effect of Solvent on Chemical Shift 322 6.8 Chemical Shift Reagents; High-Field Spectra 323 6.9 Chiral Resolving Agents 326

6.10 Spin Decoupling Methods; Double Resonance 326 6.11 NOE Difference Spectra 329

Problems 332 References 352

CHAPTER 7

ULTRAVIOLET SPECTROSCOPY 353

TA The Nature of Electronic Excitations 353 7.2 The Origin of UV Band Structure 355 73 Principles of Absorption Spectroscopy 355 74 Instrumentation 356