Magnetic Resonance Spectroscopy

Transcription

1 INTRODUCTION TO Magnetic Resonance Spectroscopy ESR, NMR, NQR D. N. SATHYANARAYANA Formerly, Chairman Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore % I.K. International Publishing House Pvt. Ltd. NEW DELHI BANGALORE

2 Contents Preface Acknowledgements vii ix 1 General Introduction 1 Introduction 1 Electromagnetic Radiation 2 Number of Spectral Lines 4 Spectral Line Width 4 Band Intensities 6 Spin Transitions 7 PART ONE: ELECTRON SPIN RESONANCE 2 Basic Theory 11 General Remarks 11 Electron Spin and Magnetic Moment 12 ESR Transitions 14 Selection Rules 16 g-factor 16 Presentation of Spectra 17 Interaction of Magnetic Dipoles with Microwave Radiation 18 Larmor Precession 19 Resonance Phenomena 20 Relaxation Processes 22 Transition Probability 24 3 Hyperfine Structure 27 Nuclear Hyperfine Splitting 27 Radicals Containing One Proton 27 Spin Hamiltonian 29 Selection Rules 32 Radicals Containing a Set of Equivalent Protons 33 Radicals Containing Multiple Sets of Protons 38 Radicals Containing Other Nuclei of Spin I = 1/2 43 Radicals Containing Nuclei with Spin I > 1/2 44 Aromatic Radicals 48 Origin of Hyperfine Interaction 49 Dipolar Interaction 49 Isotropic Hyperfine Interaction 51 Spin Polarization 53 Sigma Radicals 57 Assignment of Spectra using Huckel MOs 59

3 xii Contents Alternant Hydrocarbons 61 Hyperfine Splitting Constants 65 Second Order Splitting 67 Applications 67 Identification and Structure Elucidation 67 Study oftransient Paramagnetic Species 68 Biochemical Applications 69 Analytical Applications 69 4 Experimental Aspects: ESR 71 The ESR Spectrometer 71 Source 71 Sample Cavity 71 Magnet 72 Sampling Procedure 73 Reference Spectra 74 Determination of g-value 75 5 Spectral Characteristics: Line Width and Anisotropy 79 Line Width 79 Introduction 79 Lifetime Broadening 80 Inhomogeneous Broadening 82 Homogeneous Broadening 83 Other Factors Affecting Line Width 83 Anisotropy 83 Anisotropy in ^-Factor 83 Anisotropy in A-Values 89 6 Dynamic Processes 91 Introduction 91 General Model for Interconversion 91 Electron Spin Exchange 93 Electron Transfer 94 Proton Exchange 95 Fluxional Molecules 95 7 The Triplet State 97 General Remarks 97 Spin Transitions in Triplet State 98 Effect of Dipolar Field 99 Zero-Field Splitting 100 Spectra of Naphthalene Triplet 102 Hyperfine and Zero-Field Splitting in Triplet State Spectra Transition Metal Complexes 107

4 Contents xiii Introduction 107 Features of the Spectra of Transition Metal Complexes 108 Energy Levels in a Metal Ion 111 Russell-Saunders Coupling 111 Hund's Rules 113 Spin-Orbit Coupling 115 Effect of Crystal Field on d Orbitals 116 Effect of Crystal Field on g-values 120 Jahn-Teller and Kramers Theorems 121 Spectra of First Transition Series : A Survey 122 (a) 3d1 and3cf9ions 122 (b) 3d2and3rf8Ions 131 (c) 3</3 and 3c?7 Ions 132 (d) 3d4and3rf6Ions 135 (e) 3d5 Ion Double Resonance Techniques 140 Introduction 140 Electron-Nuclear Double Resonance 140 Electron-Electron Double Resonance 144 General References 146 Exercises 146 PART TWO: NUCLEAR MAGNETIC RESONANCE 10 General Principles 155 Nuclear Spin and Magnetic Moment 155 Resonance Frequencies 158 Population of Energy Levels 160 Larmor Precession 161 The NMR Spectrum 163 Relaxation Processes Chemical Shift 167 Shielding Constant 167 Chemical Shift 168 Measurement of Peak Intensity 169 Measurement of Chemical Shifts 170 Reference Compounds 173 Simple Applications of Chemical Shift 175 Interpreting Chemical Shift 176 Origin of Shielding Constant 178 (a) Local Diamagnetic Shielding 179 (b) Neighbouring Group Anisotropy 181

5 xiv Contents (c) Ring Current 185 (d) Local Paramagnetic Shielding 188 (e) Contact Interaction 188 (f) Hydrogen Bonding Spin-Spin Coupling 192 Scalar Coupling 192 The Energy Levels of Coupled Systems 194 First Order Spectra 196 Nomenclature for Spin Systems 197 Patterns of Coupling 198 (i) AX System 199 (ii) AX2 System 200 (iii) AX3 System 200 (iv) AX System 201 (v) AMX System 203 (vi) System with I > Observed Coupling Constants 206 Two-Bond Coupling 209 Three-Bond Coupling 210 Long-Range Coupling 211 Second Order Spectra 212 (i) AB System 212 (ii) AB2 System 216 (iii) ABX System 218 Origin of Spin-Spin Coupling 221 (i) Contact Interaction 221 (ii) Dipolar Interaction 223 Aids in the Analysis of Spectra 223 (i) Varying Magnetic field 223 (ii) Isotopic Substitution 224 (iii) Computation of Spectra Experimental Aspects: NMR 225 FT NMR Spectrometer 225 (i) Magnet 226 (ii) Radiofrequency Transmitter 229 (iii) NMR Probe 229 (iv) Computer 229 Radiofrequency Pulses 230 Theory of NMR Experiment 232 Larmor Precession and Relaxation 232 The NMR Spectrum 237 Calibration 238

6 Contents xv Advantages of FT NMR 239 Sampling Procedure 240 Variable Temperature NMR Dynamic NMR Spectroscopy 242 Introduction 242 Symmetrical Two-Site Exchange 243 Slow Exchange 244 Fast Exchange 245 Intermediate Exchange 247 Barrier to Internal Rotation 249 Unsymmetrical Two-Site Exchange 250 Ring Inversion 252 Fluxional Molecules 254 Intermolecular Exchange Processes 254 Proton Exchange 254 Intramolecular Exchange Processes 256 Keto-Enol Tautomerism 256 Fluorophosphoranes 258 Organometallic Compounds 258 Substituted Ethanes Spectra of Other Nuclei: 13C,19F and 31P 265 General Remarks 265 Carbon-13 NMR 265 Peak Assignments 267 Off-Resonance Decoupling 268 Gated Decoupling 271 Other NMR Experiments 271 Polarization Transfer Experiment 271 Attached Proton Test (APT) 275 INEPT Spectra 278 DEPT Spectra C Chemical Shifts C *H Coupling Constants F NMR 292 Chemical Shifts 292 Coupling Constants 293 Some Examples P NMR 296 Chemical Shifts 296 Coupling Constants 297 Some Examples 298 Geometrical Isomers 300 Two-Bond Coupling 301

7 ? xvi Contents Long Range Coupling Relaxation Processes 303 General Remarks 303 Spin-Lattice Relaxation 303 Spin-Spin Relaxation 306 Measurement of Relaxation Times 309 Measurement of 7\ : Inversion Recovery Method 309 Measurement of T2: Spin Echoes Method.312 Quadrupolar Relaxation 317 Effect of Quadrupolar Relaxation on the Spectrum 318 Applications of Relaxation Times Multiple Resonance Techniques 321 Homonuclear Double Resonance 321 Heteronuclear Double Resonance 323 Broad Band Decoupling 325 Off-Resonance Decoupling 326 Gated Decoupling 327 Spin Tickling 328 Sign of the Coupling Constants 330 Coupling with Low-Abundant Nuclei 332 The Nuclear Overhauser Effect 333 Intemuclear Double Resonance Selected Topics 341 Spectra of Paramagnetic Materials 341 Contact Shift 341 Origin of Contact Shift 344 Pseudo Contact Shift 347 Application of Contact Shifts 347 Diamagnetic Complexes 349 Spectra of Free Radicals 349 Lanthanide Shift Reagents 351 Magnetic Susceptibility Measurement 354 Solid State NMR 355 Wide Line NMR 355 Magic Angle Spinning 356 Applications 360 Magnetic Resonance Imaging Two-Dimensional NMR Spectroscopy 364 Introduction 364 Principles of 2D NMR 364 Preparation 366

8 13C Contents xvii Evolution 366 Mixing 366 Detection 366 2D NMR experiment 367 Presentation of 2D NMR Spectra 369 ^HCOSY 370 Modification of COSY 374 (a) COSY-DQF 374 (b) COSY (c) COSY-LR (Long Range COSY) HETCOR (le COSY) 377 (a) Heteronuclear Multiple Quantum Coherence (HMQC) 381 (b) Heteronuclear Multiple Bond Connectivity (HMBC) Resolved Spectra 384 2D Nuclear Overhauser Spectroscopy (NOESY) 385 2D INADEQUATE Spectroscopy 389 Applications of 2D NMR 394 General References 396 Exercises _ PART THREE: NUCLEAR QUADRUPOLE RESONANCE Nuclear Quadrupole Resonance Spectroscopy 411 Introduction 411 Nuclear Quadrupole Moment 412 Electric Field Gradient 414 The Asymmetry Parameter 416 Nuclear Quadrupole Transitions 419 (a) Axially Symmetric Molecules 419 (b) Axially Non-Symmetric Molecules 422 Effect of an External Magnetic Field 425 Applications 427 (i) Chemical Bonding and Structure 427 (ii) Solid State Effects 430 (iii) Hydrogen Bonding 433 Experimental Aspects 433 General References 435 Exercises 435 Appendix General Data and Fundamental Constants 437 Index