1 The Physical Basis of NMR Spectroscopy ....................... 1
1.1 Introduction ............................................ 1
1.2 Nuclear Angular Momentum and Magnetic Moment ............ 2
1.3 Nuclei in a Static Magnetic Field ....................... 4
1.3.1 Directional Quantization ......................... 4
1.3.2 Energy of the Nuclei in the Magnetic Field ....... 4
1.3.3 Populations of the Energy Levels ................. 6
1.3.4 Macroscopic Magnetization ........................ 6
1.4 Basic Principles of the NMR Experiment .................. 7
1.4.1 The Resonance Condition .......................... 7
1.4.2 Basic Principle of the NMR Measurement ........... 8
1.5 The Pulsed NMR Method ................................... 9
1.5.1 The Pulse ........................................ 9
1.5.2 The Pulse Angle ................................. 10
1.5.3 Relaxation ...................................... 13
1.5.4 The Time and Frequency Domains; the Fourier
Transformation .................................. 14
1.5.5 Spectrum Accumulation ........................... 16
1.5.6 The Pulsed NMR Spectrometer ..................... 18
1.6 Spectral Parameters: a Brief Survey .................... 22
1.6.1 The Chemical Shift .............................. 22
1.6.1.1 Nuclear Shielding ...................... 22
1.6.1.2 Reference Compounds and the (5-Scale ... 24
1.6.2 Spin-Spin Coupling .............................. 26
1.6.2.1 The Indirect Spin-Spin Coupling ........ 26
1.6.2.2 Coupling to One Neighboring Nucleus
(AX Spin System) ....................... 27
1.6.2.3 Coupling to Two Equivalent
Neighboring Nuclei (AX2 Spin System) ... 29
1.6.2.4 Coupling to Three or More Equivalent
Neighboring Nuclei (AXn Spin System) ... 30
1.6.2.5 Multiplicity Rules ..................... 30
1.6.2.6 Couplings between Three
Non-equivalent Nuclei (AMX Spin
System) ................................ 31
1.6.2.7 Couplings between Equivalent Nuclei
(An Spin Systems) ...................... 32
1.6.2.8 The Order of a Spectrum ................ 33
1.6.2.9 Couplings between Protons and other
Nuclei; 13C Satellite Spectra .......... 33
1.6.3 The Intensities of the Resonance Signals ........ 34
1.6.3.1 1H Signal Intensities .................. 34
1.6.3.2 13C Signal Intensities ................. 35
1.6.4 Summary ......................................... 37
1.7 "Other" Nuclides ....................................... 38
1.7.1 Nuclides with Spin I = 1/2 ...................... 39
1.7.2 Nuclides with Spin I > 1/2 ...................... 40
Exercises ................................................... 41
1.8 Bibliography for Chapter 1 ............................. 41
2 The Chemical Shift .......................................... 43
2.1 Introduction ........................................... 43
2.1.1 Influence of the Charge Density on the
Shielding ....................................... 44
2.1.2 Effects of Neighboring Groups ................... 47
2.1.2.1 Magnetic Anisotropy of Neighboring
Groups ................................. 47
2.1.2.2 Ring Current Effects ................... 49
2.1.2.3 Electric Field Effects ................. 51
2.1.2.4 Intermolecular Interactions -
Hydrogen Bonding and Solvent Effects ... 51
2.1.2.5 Isotope Effects ........................ 51
2.1.3 Summary ......................................... 52
2.2 1H Chemical Shifts of Organic Compounds ................ 53
2.2.1 Alkanes and Cycloalkanes ........................ 54
2.2.2 Alkenes ......................................... 56
2.2.3 Arenes .......................................... 56
2.2.4 Alkynes ......................................... 57
2.2.5 Aldehydes ....................................... 58
2.2.6 OH, SH, NH ...................................... 59
2.3 13C Chemical Shifts of Organic Compounds ................ 60
2.3.1 Alkanes and Cycloalkanes ........................ 61
2.3.2 Alkenes ......................................... 63
2.3.3 Arenes .......................................... 64
2.3.4 Alkynes ......................................... 66
2.3.5 Allenes ......................................... 66
2.3.6 Carbonyl and Carboxy Compounds .................. 66
2.3.6.1 Aldehydes and Ketones .................. 67
2.3.6.2 Carboxylic Acids and Derivatives ....... 68
2.4 Relationships between the Spectrum and the Molecular
Structure .............................................. 70
2.4.1 Equivalence, Symmetry and Chirality ............. 70
2.4.2 Homotopic, Enantiotopic and Diastereotopic
Groups .......................................... 74
2.4.3 Summary ......................................... 77
2.5 Chemical Shifts of "Other" Nuclides .................... 78
Exercises ................................................... 83
2.6 Bibliography for Chapter 2 ............................. 83
3 Indirect Spin-Spin Coupling ................................. 85
3.1 Introduction ........................................... 85
3.2 H,H Coupling Constants and Chemical Structure .......... 87
3.2.1 Geminal Couplings 2J(H,H) ....................... 87
3.2.1.1 Dependence on Bond Angle ............... 87
3.2.1.2 Substituent Effects .................... 88
3.2.1.3 Effects of Neighboring π-Electrons ..... 88
3.2.2 Vicinal Couplings 3J(H,H) ....................... 89
3.2.2.1 Dependence on the Dihedral Angle ....... 90
3.2.2.2 Substituent Effects .................... 94
3.2.3 H,H Couplings in Aromatic Compounds ............. 95
3.2.4 Long-range Couplings ............................ 96
3.3 C,H Coupling Constants and Chemical Structure .......... 97
3.3.1 C,H Couplings through One Bond 1J(C,H) .......... 97
3.3.1.1 Dependence on the s-Fraction ........... 97
3.3.1.2 Substituent Effects .................... 98
3.3.2 C,H Couplings through Two or More Bonds ......... 99
3.3.2.1 Geminal Couplings (i.e. 2J(C,H)
in H-C-13C) ............................ 99
3.3.2.2 Vicinal Couplings (i.e. 3J(C,H)
in H-C-C-13C) .......................... 99
3.3.2.3 Long-range Couplings 3+nJ(C,H) ......... 100
3.3.3 C,H Couplings in Benzene Derivatives ........... 100
3.4 C,C Coupling Constants and Chemical Structure ......... 101
3.5 Correlations between C,H and H,H Coupling Constants ... 101
3.6 Coupling Mechanisms ................................... 103
3.6.1 The Electron-Nuclear Interaction ............... 103
3.6.2 H,D Couplings .................................. 105
3.6.3 Relationship between the Coupling and the
Lifetime of a Spin State ....................... 106
3.6.4 Couplings through Space ........................ 106
3.7 Couplings of "Other" Nuclides (Heteronuclear
Couplings) ............................................ 107
Exercises .................................................. 109
3.8 Bibliography for Chapter 3 ............................ 109
4 Spectrum Analysis and Calculations ......................... 111
4.1 Introduction .......................................... 111
4.2 Nomenclature .......................................... 113
4.2.1 Systematic Notation for Spin Systems ........... 113
4.2.2 Chemical and Magnetic Equivalence .............. 114
4.3 Two-Spin Systems ...................................... 116
4.3.1 The AX Spin System ............................. 116
4.3.2 The AB Spin System ............................. 118
4.4 Three-Spin Systems .................................... 120
4.4.1 The AX2, AK2, AB2 and A3 Spin Systems ........... 120
4.4.2 The AMX and ABX Spin Systems ................... 121
4.5 Four-Spin Systems ..................................... 123
4.5.1 A2X2 and A2B2 Spin Systems ...................... 123
4.5.2 The AA'XX' and AA'BB' Spin Systems ............. 124
4.6 Spectrum Simulation and Iteration ..................... 125
4.7 Analysis of 13C NMR Spectra ........................... 126
Exercises .................................................. 127
4.8 Bibliography for Chapter 4 ............................ 127
5 Double Resonance Experiments ............................... 129
5.1 Introduction .......................................... 129
5.2 Spin Decoupling in 1H NMR Spectroscopy ................ 130
5.2.1 Simplification of Spectra by Selective Spin
Decoupling ..................................... 130
5.2.2 Suppression of a Solvent Signal ................ 132
5.3 Spin Decoupling in 13C NMR Spectroscopy ............... 133
5.3.1 1H Broad-band Decoupling ....................... 133
5.3.2 The Gated Decoupling Experiment ................ 135
5.3.3 1H Off-Resonance Decoupling .................... 136
5.3.4 Selective Decoupling in 13C NMR Spectroscopy ... 137
Exercises .................................................. 138
5.4 Bibliography for Chapter 5 ............................ 138
6 Assignment of 1H and 13C Signals ............................ 139
6.1 Introduction .......................................... 139
6.2 1H NMR Spectroscopy ................................... 140
6.2.1 Defining the Problem ........................... 140
6.2.2 Empirical Correlations for Predicting
Chemical Shifts ................................ 141
6.2.2.1 Alkanes (Shoolery's Rule) ............. 141
6.2.2.2 Alkenes ............................... 142
6.2.2.3 Benzene Derivatives ................... 143
6.2.3 Decoupling Experiments ......................... 145
6.2.4 Altering the Chemical Structure of the
Sample ......................................... 145
6.2.5 Effects of Solvent and Temperature ............. 146
6.2.6 Shift Reagents ................................. 147
6.2.6.1 Lanthanide Shift Reagents (LSRs) ...... 147
6.2.6.2 Chiral Lanthanide Shift Reagents ...... 150
6.3 13C NMR Spectroscopy .................................. 152
6.3.1 Defining the Problem ........................... 152
6.3.2 Empirical Correlations for Predicting
Approximate Chemical Shifts .................... 154
6.3.2.1 Alkanes ............................... 154
6.3.2.2 Alkenes ............................... 157
6.3.2.3 Benzene Derivatives ................... 158
6.3.3 Decoupling Experiments ......................... 159
6.3.4 T1 Measurements ................................ 160
6.3.5 Chemical Changes to the Sample ................. 160
6.3.6 Solvent and Temperature Effects and Shift
Reagents ....................................... 161
6.4 Computer-aided Assignment of 13C NMR Spectra .......... 161
6.4.1 Searching for Identical or Related Compounds ... 161
6.4.2 Spectrum Prediction ............................ 162
Exercises .................................................. 164
6.5 Bibliography for Chapter 6 ............................ 165
7 Relaxation ................................................. 167
7.1 Introduction .......................................... 167
7.2 Spin-Lattice Relaxation of 13C Nuclei (T1) ............. 168
7.2.1 Relaxation Mechanisms .......................... 168
7.2.2 Experimental Determination of T1; the
Inversion Recovery Experiment .................. 170
7.2.3 Relationships between T1 and Chemical
Structure ...................................... 174
7.2.3.1 Influence of Protons in CH, CH2 and
CH3 Groups ............................ 174
7.2.3.2 Influence of Molecular Size ........... 175
7.2.3.3 Segmental Mobilities .................. 176
7.2.3.4 Anisotropy of the Molecular
Mobility .............................. 176
7.2.4 Suppression of the Water Signal ................ 177
7.3 Spin-Spin Relaxation (T2) ............................. 177
7.3.1 Relaxation Mechanisms .......................... 177
7.3.2 Experimental Determination of Т2
the Spin-Echo Experiment ....................... 179
7.3.3 Line-widths of NMR Signals ..................... 183
Exercises .................................................. 185
7.4 Bibliography for Chapter 7 ............................ 185
8 One-Dimensional NMR Experiments using Complex Pulse
Sequences .................................................. 187
8.1 Introduction .......................................... 187
8.2 Basic Techniques Using Pulse Sequences and Pulsed
Field Gradients ....................................... 188
8.2.1 The Effect of the Pulse on the Longitudinal
Magnetization (Mz) ............................. 189
8.2.2 The Effect of the Pulse on the Transverse
Magnetization Components (Mx; My) .............. 190
8.2.3 Spin-Locking ................................... 193
8.2.4 The Effect of Pulsed Field Gradients on the
Transverse Magnetization ....................... 195
8.3 The J-Modulated Spin-Echo Experiment .................. 199
8.4 The Pulsed Gradient Spin-Echo Experiment .............. 208
8.5 Signal Enhancement by Polarization Transfer ........... 210
8.5.1 The SPI Experiment ............................. 210
8.5.2 The INEPT Experiment ........................... 213
8.5.3 The Reverse INEPT Experiment with Proton
Detection ...................................... 221
8.6 The DEPT Experiment ................................... 226
8.7 The Selective TOCSY Experiment ........................ 230
8.8 The One-Dimensional INADEQUATE Experiment ............. 233
Exercises ............................................. 237
8.9 Bibliography for Chapter 8 ............................ 237
9 Two-Dimensional NMR Spectroscopy ........................... 239
9.1 Introduction .......................................... 239
9.2 The Two-Dimensional NMR Experiment .................... 240
9.2.1 Preparation, Evolution and Mixing, Data
Acquisition .................................... 240
9.2.2 Graphical Representation ....................... 244
9.3 Two-Dimensional J-Resolved NMR Spectroscopy ........... 245
9.3.1 Heteronuclear Two-Dimensional J-Resolved NMR
Spectroscopy ................................... 245
9.3.2 Homonuclear Two-Dimensional J-Resolved NMR
Spectroscopy ................................... 249
9.4 Two-Dimensional Correlated NMR Spectroscopy ........... 254
9.4.1 Two-Dimensional Heteronuclear (CH)-Correlated
NMR Spectroscopy (HETCOR or C,H-COSY) .......... 255
9.4.2 Two-Dimensional Homonuclear (H,H)-Correlated
NMR Spectroscopy (H,H-COSY; Long-Range COSY) ... 263
9.4.3 Reverse Two-Dimensional Heteronuclear (H,C)-
Correlated NMR Spectroscopy (HSQC; HMQC) ....... 271
9.4.4 The Gradient-Selected (gs-)HMBC Experiment ..... 276
9.4.5 The TOCSY Experiment ........................... 281
9.4.6 Two-Dimensional Exchange NMR Spectroscopy:
The Experiments NOESY ROESY and EXSY ........... 284
9.5 The Two-Dimensional INADEQUATE Experiment ............. 289
9.6 Summary of Chapters 8 and 9 ........................... 294
Exercises ............................................. 295
9.7 Bibliography for Chapter 9 ............................ 295
10 The Nuclear Overhauser Effect .............................. 297
10.1 Introduction .......................................... 297
10.2 Theoretical Background ................................ 298
10.2.1 The Two-Spin System ............................ 298
10.2.2 Enhancement Factors ............................ 301
10.2.3 Multi-Spin Systems ............................. 302
10.2.4 From the One-Dimensional to the Two-
Dimensional Experiments, NOESY and ROESY ....... 303
10.3 Experimental Aspects .................................. 305
10.4 Applications .......................................... 306
Exercises ............................................. 311
10.5 Bibliography for Chapter 10 ........................... 311
11 Dynamic NMR Spectroscopy (DNMR) ............................ 313
11.1 Introduction .......................................... 313
11.2 Quantitative Calculations ............................. 317
11.2.1 Complete Line-shape Analysis ................... 317
11.2.2 The Coalescence Temperature Tc and the
Corresponding Rate Constant kc ................. 319
11.2.3 Activation Parameters .......................... 320
11.2.3.1 The Arrhenius Activation Energy ЕA .... 320
11.2.3.2 The Free Enthalpy of Activation ΔG .... 321
11.2.3.3 Estimating the Limits of Error ........ 322
11.2.4 Rate Constants in Reactions with Intermediate
Stages ......................................... 323
11.2.5 Intermolecular Exchange Processes .............. 324
11.3 Applications .......................................... 325
11.3.1 Rotation about CC Single Bonds ................. 325
11.3.1.1 C(sp3)-C(sp3) Bonds .................... 326
11.3.1.2 C(sp2)-C(sp3) Bonds .................... 326
11.3.1.3 C(sp2)-C(sp2) Bonds .................... 327
11.3.2 Rotation about a Partial Double Bond ........... 327
11.3.3 Inversion at Nitrogen and Phosphorus Atoms ..... 329
11.3.4 Ring Inversion ................................. 330
11.3.5 Valence Tautomerism ............................ 333
11.3.6 Keto-Enol Tautomerism .......................... 334
11.3.7 Intermolecular Proton Exchange ................. 335
11.3.8 Reactions and Equilibration Processes .......... 337
Exercises .................................................. 340
11.4 Bibliography for Chapter 11 ........................... 340
12 Synthetic Polymers ......................................... 343
12.1 Introduction .......................................... 343
12.2 The Tacticity of Polymers ............................. 343
12.3 Polymerization of Dienes .............................. 347
12.4 Copolymers ............................................ 348
12.5 Solid-State NMR Spectroscopy of Polymers .............. 349
Exercises ............................................. 352
12.6 Bibliography for Chapter 12 ........................... 352
13 NMR Spectroscopy in Biochemistry and Medicine .............. 355
13.1 Introduction .......................................... 355
13.2 Elucidating Reaction Pathways in Biochemistry ......... 355
13.2.1 Syntheses using Singly 13C-Labeled
Precursors ..................................... 356
13.2.1.1 Low Levels of 13C Enrichment .......... 356
13.2.1.2 High Levels of 13C Enrichment ......... 357
13.2.2 Syntheses using Doubly 13C-Labeled
Precursors ..................................... 358
13.3 Biopolymers .......................................... 360
13.3.1 Peptides and Proteins .......................... 361
13.3.1.1 Sequence Analysis ..................... 362
13.3.1.2 The Three-Dimensional Structure of
Proteins .............................. 363
13.3.2 Polynucleotides ................................ 365
13.3.3 Oligosaccharides and Polysaccharides ........... 367
13.4 Saturation Transfer Difference NMR Spectros
copy (STD) ............................................ 371
Exercises .................................................. 372
13.5 Bibliography for Chapter 13 ........................... 372
14 In vivo NMR Spectroscopy in Biochemistry and Medicine ...... 375
14.1 Introduction .......................................... 375
14.2 High-Resolution in vivo NMR Spectroscopy .............. 376
14.2.1 The Problem and its Solution ................... 376
14.2.2 31P NMR Experiments ............................ 377
14.2.3 1H and 13C NMR Experiments ...................... 380
14.3 Magnetic Resonance Tomography ......................... 381
14.3.1 Basic Principles and Experimental
Considerations ................................. 381
14.3.2 Applications ................................... 387
14.4 Magnetic Resonance Spectroscopy, 1H MRS ............... 391
Exercises ............................................. 393
14.5 Bibliography for Chapter 14 ........................... 393
Solutions .................................................. 395
Subject Index ................................................. 407
Index of Compounds ............................................ 411
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