Organic Structures from Spectra

Preface ix
List of Tables xiii
List of Figures xv

1 Introduction 1
1.1 General Principles of Absorption Spectroscopy 1
1.2 Chromophores 2
1.3 Degree of Unsaturation 3
1.4 Connectivity 4
1.5 Sensitivity 4
1.6 Practical Considerations 5

2 Ultraviolet (UV) Spectroscopy 6
2.1 The Nature of Ultraviolet Spectroscopy 6
2.2 Basic Instrumentation 6
2.3 Quantitative Aspects of Ultraviolet Spectroscopy 8
2.4 Classification of UV Absorption Bands 8
2.5 Special Terms in Ultraviolet Spectroscopy 9
2.6 Important UV Chromophores 10
2.6.1 Dienes and Polyenes 10
2.6.2 Carbonyl Compounds 11
2.6.3 Benzene Derivatives 11
2.7 The Effect of Solvents 13

3 Infrared (IR) Spectroscopy 14
3.1 Absorption Range and the Nature of IR Absorption 14
3.2 Experimental Aspects of Infrared Spectroscopy 15
3.3 General Features of Infrared Spectra 16
3.4 Important IR Chromophores 18
3.4.1 –O–H and –N–H Stretching Vibrations 18
3.4.2 C–H Stretching Vibrations 18
3.4.3 –C≡N and –C≡C– Stretching Vibrations 19
3.4.4 Carbonyl Groups 19
3.4.5 Other Polar Functional Groups 21
3.4.6 The Fingerprint Region 21

4 Mass Spectrometry 23
4.1 Ionisation Processes 23
4.2 Instrumentation 25
4.3 Mass Spectral Data 26
4.3.1 High Resolution Mass Spectra 26
4.3.2 Molecular Fragmentation 28
4.3.3 Isotope Ratios 29
4.3.4 Chromatography Coupled With Mass Spectrometry 31
4.3.5 Metastable Peaks 31
4.4 Representation of Fragmentation Processes 31
4.5 Factors Governing Fragmentation Processes 32
4.6 Examples of Common Types of Fragmentation 32
4.6.1 Cleavage at Branch Points 32
4.6.2 Β-Cleavage 33
4.6.3 Cleavage Α to Carbonyl Groups 33
4.6.4 Cleavage Α to Heteroatoms 34
4.6.5 Retro Diels–Alder Reaction 34
4.6.6 The Mclafferty Rearrangement 34

5 1H Nuclear Magnetic Resonance (NMR) Spectroscopy 36
5.1 The Physics of Nuclear Spins and NMR Instruments 36
5.1.1 The Larmor Equation and Nuclear Magnetic Resonance 36
5.2 Basic NMR Instrumentation 39
5.2.1 CW and Pulsed NMR Spectrometers 39
5.2.2 Nuclear Relaxation 42
5.2.3 Magnets for NMR Spectroscopy 43
5.2.4 The NMR Spectrum 44
5.3 Chemical Shift in 1H NMR Spectroscopy 45
5.4 Spin–Spin Coupling in 1H NMR Spectroscopy 52
5.4.1 Signal Multiplicity – The N+1 Rule 54
5.5 Analysis of 1H NMR Spectra 55
5.5.1 Spin Systems 56
5.5.2 Strongly and Weakly Coupled Spin Systems 56
5.5.3 Magnetic Equivalence 58
5.5.4 Conventions for Naming Spin Systems 59
5.5.5 Spectral Analysis of First-Order NMR Spectra 60
5.5.6 Splitting Diagrams 61
5.5.7 Spin Decoupling 64
5.6 Correlation of 1H–1H Coupling With Structure 65
5.6.1 Non-Aromatic Spin Systems 65
5.6.2 Aromatic Spin Systems 66
5.7 The Nuclear Overhauser Effect (NOE) 69
5.8 Labile and Exchangeable Protons 70

6 13c NMR Spectroscopy 72
6.1 Coupling and Decoupling in 13c NMR Spectra 72
6.2 The Nuclear Overhauser Effect (NOE) in 13c NMR Spectroscopy 73
6.3 Determining 13c Signal Multiplicity Using Dept 73
6.4 Shielding and Characteristic Chemical Shifts in 13c NMR Spectra 76

7 2-Dimensional NMR Spectroscopy 82
7.1 Proton–Proton Interactions By 2D NMR 85
7.1.1 COSY (Correlation Spectroscopy) 85
7.1.2 TOCSY (Total Correlation Spectroscopy) 86
7.1.3 NOESY (Nuclear Overhauser Effect Spectroscopy) 88
7.2 Proton–Carbon Interactions By 2D NMR 89
7.2.1 The HSQC (Heteronuclear Single Quantum Correlation) or HSC (Heteronuclear Shift Correlation) Spectrum 89
7.2.2 HMBC (Heteronuclear Multiple Bond Correlation) 91

8 Miscellaneous Topics 96
8.1 Solvents for NMR Spectroscopy 96
8.2 Solvent-Induced Shifts 97
8.3 Dynamic Processes in NMR – The NMR Time-Scale 98
8.3.1 Conformational Exchange Processes 99
8.3.2 Intermolecular Exchange of Labile Protons 99
8.3.3 Rotation About Partial Double Bonds 100
8.4 The Effect of Chirality 100
8.5 The NMR Spectra of “Other Nuclei” 101

9 Determining the Structure of Organic Compounds From Spectra 102
9.1 Solving Problems 103
9.2 Worked Examples 104

10 Problems 115

Index 538