A Complete Introduction to MODERIN NMR Spectroscopy Roger s Macomber
Dedication First, to those closest to me: my parents, Roxanne, Barbara, Dan, and Juliann. And second, to the memory of Thomas L. Jacobs of UCLA, the person who inspired my life-long interest in organic chemistry. Without all of you, this book would never have tto being
CONTENTS Preface Frequently Used Symbols and Abbreviations Chapter 1 SPECTROSCOPY: SOME PRELIMINARY CONSIDERATIONS 1. I What is NMR Spectroscopy? erties of Electromagnetic Radiation 1.3 Interaction of Radiation with Matter: The Classical Picture 1. 4 Uncertainty and the Question of Time Scale Chapter Summary Review Problems Chapter 2 MAGNETIC PROPERTIES OF NUCLEI 2.1 The structure of an atom 2.2 The Nucleus in a Magnetic Field 669 2.3 Nuclear Energy Levels and Relaxation Times 2.4 The Rotating Frame of Reference 2.5 Relaxation Mechanisms and Correlation Times Chapter Summary Additional Resou Review Problems Chapter 3 OBTAINING AN NMR SPECTRUM 3. 1 Electricity and Magnetism 3.2 The NMR Magnet 3.3 Signal Generation the Old Way: The Continuous-Wave(CW) Experiment 3.4 The Modern Pulsed Mode for Signal Acquisition 3.5 Line Widths, Lineshape, and Sampling Considerations 3.6 Measurement of Relaxation Times
vi CONTENTS Additional Resources Review Problems Chapter 4 A LITTLE BIT OF SYMMETRY 4.1 Symmetry Operations and Distinguishability 4. 2 Conformations and Their Symmetry 4.3 Homotopic, Enantiotopic, and Diastereotopic Nucle 4.4 Accidental Equivalence Chapter Summary Additional Resources 4534444 Review Problems Chapter 5 THE H ANDC NMR SPECTRA OF TOLUENE 5.1 The H NMR Spectrum of Toluene at 80 MHz 5.2 The Chemical Shift Scale 5.3 The 250-and 400-MHz 'H NMR Spectra of Toluene 690 5.4 The"C NMR Spectrum of Toluene at 20.1, 62.9, and 100.6 MHz 5.5 Data Acquisition Parameters 64 Review Problems 67 Chapter 6 CORRELATING PROTON CHEMICAL SHIFTS WITH MOLECULAR STRUCTURE 68 6. 1 Shielding and Deshielding 6.2 Chemical Shifts of Hydrogens Attached to Tetrahedral Carbon 6.3 Vinyl and Formyl Hydrogen Chemical Shifts 6.4 Magnetic Anisotropy 6.5 Aromatic Hydrogen Chemical Shift Correlations 6.6 Hydrogen Attached to Elements Other than Carbon Chapter Summary References 804788566 Additional Resources Review Problems Chapter7 CHEMICAL SHIFT CORRELATIONS FORC AND OTHER ELEMENTS 7. 1 3C Chemical Shifts Revisited 88 7.2 Tetrahedral (sp Hybridized) Carbons 88 7.3 Heterocyclic Structures 7. 4 Trigonal Carbons 92 7.5 Triply Bonded Carbons 7.6 Carbonyl Carbons 96 7.7 Miscellaneous Unsaturated Carbons 7. 8 Summary ofC Chemical Shifts 7.9 Chemical Shifts of Other Elements 10l Chapter Summary References 102 Review Problems SELF-TEST I Chapter 8 FIRST-ORDER (WEAK)SPIN-SPIN COUPLING 110 8.1 Unexpected Lines in an NMR Spectrum l10
8.2 The ' H Spectrum of Diethyl Ether l10 8.4 The Spin-Spin Coupling Checkllisrplified Picture 8.3 Homonuclear H Coupling: The Sin I 12 13 8.5 The n+I Rule l14 8.6 Heteronuclear Spin-Spin Coupling l17 8.7 Review Examples Chapter Summary Chapter 9 FACTORS THAT INFLUENCE THE SIGN AND MAGNITUDE OF J: SECOND-ORDER (STRONG) COUPLING EFFECTS 9. 1 Nuclear Spin Energy Diagrams and the Sign of 9.2 Factors that Influence J: Preliminary Considerati 9.3 One-Bond Coupling Constants 136 9.4 Two-Bond( Geminal)Coupling Constants 9.5 Three-Bond(Vicinal)Coupling Constants 138 9.6 Long-Range Coupling Constants .7 Magnetic Equivalence 143 9.8 Pople Spin System Notation 9.9 Slanting Multiplets and Second-Order(Strong Coupling) Effects 145 9.10 Calculated Spectra 9l1 The Ax→AB→A2 Continuum 9.12 More About the ABX System: Deceptive Simplicity and Virtual Coupling Chapter Summary References Review Problems Chapter 10 THE STUDY OF DYNAMIC PROCESSES BY NMR 10. 1 Reversible and Irreversible Dynamic Processes 10.2 Reversible Intramolecular Processes Involving Rotation Around Bonds 159 10.3 Simple Two-Site Intramolecular Exchange 10.4 Reversible Intramolecular Chemical Processes 10.5 Reversible Intermolecular Chemical Processes 10.6 Reversible Intermolecular Complexation 10.7 Other Examples of Reversible Complexation: Chemical Shift Reagents Chapter Summary 171 References Review Problems 172 Chapter 11 ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY AND CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION 176 11.1 Electron Paramagnetic Resonance 176 11.2 Free Radicals 176 1 1.3 The g Factor 11.4 Sensitivity Considerations 1 1.5 Hyperfine Coupling and the a value 11.6 A Typical EPR Spectrum 181 11.7 CIDNP: Mysterious Behavior of NMR Spectrometers 182 11. 8 The Net Effect 11.9 The Multiplet Effect 11.10 The Radical-Pair Theory of The Net Effect I1.11 The Radical-Pair Theory of the Multiplet Effect 187