Preface, x
Determining Structures — How and Why, 1
1.1 General introduction, 1
;
1.2 Answering questions about structure, 2
1.3 Plan of the book, 7
1.4 Timescales, 9
1.5 Modern technology and the development of instruments, 12
1.5.1 Sources, 13
1.5.2 Detectors, 15
1.5.3 Electronics and computers, 16
1.5.4 Fourier transform techniques, 17
1.6 Glossary of spectroscopic and structural techniques, 19
Nuclear Magnetic Resonance Spectroscopy, 28
2.1 Introduction, 28
2.2 The nuclear magnetic resonance phenomenon, 29
2.3 Experimental methods, 31
2.3.1 NMR spectrometers, 31
2.3.2 Continuous wave spectra, 34
2.3.3 Fourier transform spectra, 34
2.4 Information from chemical shifts, 36
2.4.1 General principles, 36
2.4.2 Proton chemical shifts, 37
2.4.3 Chemical shifts of other elements, 39
2.5 Information from NMR signal intensities, 43
2.6 Simple patterns due to coupling between spinning nuclei, 44
2.6.1 General considerations, 44
2.6.2 First order specta of spin 1/2 isotopes of 100% abundance, 45
2.6.3 Effects of spin 1/2 isotopes of low abundance, 46
2.6.4 Spectra of spin 1/2 isotopes of low abundance, 48
2.6.5 Coupling to quadrupolar nuclei, 51
2.7 Information from coupling constants, 53
2.7.1 General principles, 53
2.7.2 One-bond coupling, 53
2.7.3 Two-bond coupling, 54
2.7.4 Coupling over three or more bonds, 55
2.8 Not-so-simple spectra, 56
2.8.1 Second order spectra, 56
2.8.2 Chiral and prochiral non-equivalence, 58
2.8.3 Coincidences, 59
2.8.4 The multi-nuclear approach, 60
2.9 Relaxation, 61
2.9.1 General principles, 61
2.9.2 Relaxation mechanisms, 61
2.10 Multiple resonance, 62
2.10.1 Introduction, 62
2.10.2 Low power irradiation — population transfer, 63
2.10.3 Medium power irradiation — spin tickling, 64
2.10.4 High power irradiation — spin decoupling, 66
SQuis ts
4
2.10.5 Triple resonance, 68
2.10.6 The Nuclear Overhauser Effect, 69
2.10.7 Gated decoupling, 70
2.11 Multi-pulse methods, 70
2.11.1 Introduction, 70
2.11.2 Sensitivity enhancement, 71
2.11.3 Selective observation, 71
2.11.4 Two-dimensional NMR spectroscopy, 72
2.12 Gases, 78
2.13 Liquid crystals, 78
2.14 Solids, 81
2.15 Monitoring reactions, 87
2.15.1 Concentration measurements, 87
2.15.2 Exchange reactions, 89
2.16 Paramagnetic compounds, 92
Electron Spin and Nuclear Quadrupole Resonance
Spectroscopy, 105
3.1 The electron spin resonance experiment, 105
3.2 Hyperfine coupling in isotropic systems, 107
3.3 Anisotropic systems, 110
3.3.1 Hyperfine splittings and g factors, 110
3.3.2 Electron—electron interactions, 112
3.4 Transition metal complexes, 114
3.5 Multiple resonance, 117
3.6 The nuclear quadrupole resonance experiment, 119
3.7 Structural information from NQR spectra, 121
3.8 Interpretation of nuclear quadrupole coupling constants, 124
Rotational Spectra and Rotational Structure, 129
4.1 Introduction, 129
4.2 Rotation of molecules, 129
4.2.1 Classical rotation, 129
4.2.2 Quantization of rotational angular momentum, 130
4.2.3 Rotation constants and moments of inertia, 131
4.2.4 Centrifugal distortion; the semi-rigid rotor, 133
4.2.5 Degeneracy of rotational levels, 134
4.2.6 Nuclear spin statistics, 134
4.2.7 Nuclear quadrupole effects, 136
4.2.8 The Stark effect, 136
4.3 Rotational selection rules, 136
4.3.1 Pure rotation spectra, 137
4.3.2 Vibration—rotation spectra, 139
4.3.3 Electronic spectra, 143
4.4 Instrumental, 144
4.4.1 Pure rotational absorption spectroscopy, 144
4.4.2 High resolution Raman spectroscopy, 145
4.4.3 High resolution mid-IR spectroscopy, 145
4.4.4 High resolution electronic spectroscopy, 146
4.4.5 Tunable laser and double resonance experiments, 146
4.5 Using the information in a spectrum, 147
4.5.1 Fingerprinting, 147
4.5.2 Determination of rotation constants, 149
4.5.3 Determination of centrifugal distortion constants, 150
4.5.4 Isotopic substitution, 150
4.6 Using rotation constants to define molecular structures, 151
4.6.1 General; rp and r, structures, 151
4.6.2 Partial structures; 7, structures, 154
CONTENTS
4.6.3 Use of spectroscopically calculated corrections; r, structures, 155
4.7 Effects of low frequency vibrations, 156
4.8 Non-rigidity, 158
Vibrational Spectroscopy, 164
5.1 Introduction, 164
5.2 The physical basis — molecular vibrations, 164
5.2.1 Vibrational motions and energies, 164
5.2.2 Number of vibrational motions, 165
5.2.3 Non-ideal restoring forces — anharmonicity, 166
5.3 Observing molecular vibrations, 167
5.3.1 Absorption in the infra-red, 167
5.3.2 Raman scattering, 169
5.3.3 Information from electronic spectra, 170
5.3.4 Inelastic scattering of neutrons and electrons, 171
5.4 Spectrometers, 172
5.5 Sample handling, 173
5.5.1 Infra-red, 173
5.5.2 Raman, 176
5.6 Effects of phase on spectra, 178
5.7 Vibrational spectra and symmetry, 181
5.7.1 Fundamental vibrational selection rule, 181
5.7.2 Symmetry selection rules, 181
5.7.3 Symmetry of vibrations, 182
5.7.4 Symmetry of an entire set of normal vibrations, 187
5.8 Assignment of bands to vibrations, 190
5.8.1 Introduction, 190
5.8.2 Raman polarization, 190
5.8.3 Band contours in gases, 192
5.8.4 Intensities of allowed fundamentals, 200
5.8.5 Mode numbering, 200
5.8.6 Non-fundamental transitions, 202
5.9 Structural information from vibrational spectra, 206
5.10 Fingerprints, 207
§.10.1 Comparison with standard spectra, 207
5.10.2 Detection and identification of impurities, 207
5.10.3 Quantitative analysis of mixtures, 208
5.11 Group frequencies, 209
5.11.1 Characteristic bond stretching frequencies, 210
5.11.2 Characteristic deformation (angle bending) frequencies, 211
5.11.3 Characteristic patterns of group frequencies, 213
5.11.4 Group frequencies and types of ligand binding, 216
5.12 Use of isotopes in interpreting vibrational spectra, 217
5.12.1 H/D substitution, 218
5.12.2 Heavy-atom isotope substitution, 218
5.13 Complete empirical assignment of vibrational spectra, 222
5.14 Normal co-ordinate analysis, 225
5.15 Changes of structure with phase, 227
5.16 Vibrational spectroscopy of unstable molecules, 227
5.16.1 Flow, 228
5.16.2 Flash, 229
5.16.3 Freeze, 230
5.17 Resonance Raman spectroscopy, 232
Electronic and Photoelectron Spectroscopy, 239
6.1 Introduction, 239
6.2 Excitation and ejection of electrons, 239
6.3 Electronic energy levels in atoms and molecules, 240
CONTENTS
Vii
6.4
6.5
6.6
6.7
6.8
Core-level photoelectron spectroscopy, 243
6.4.1 Experimental method, 243
6.4.2 Core-level photoionization process, 244
6.4.3 Spin complications, 246
6.4.4 Depth effects in solid samples, 247
6.4.5 Chemical shifts, 247
6.4.6 Comparison of chemical shifts with theory, 249
Symmetry and molecular orbitals, 251
Valence-electron photoelectron spectroscopy, 253
6.6.1 Instrumental, 253
6.6.2 Vibrational structure of PE bands, 254
6.6.3 Structural information from valence shell PES, 260
6.6.4 Spin—orbit coupling, 261
6.6.5 Jahn-Teller distortions, 262
6.6.6 Valence photoelectron spectra of transition metal complexes, 262
Valence excitation spectroscopy, 263
6.7.1 Experimental methods, 263
6.7.2 The information in an electronic spectrum, 265
6.7.3 Calculation of valence excitation energies, 269
Electronic spectra of transition metal complexes, 269
6.8.1 Metal, ligand and metal—ligand bonding levels, 269
6.8.2 Ligand-—ligand transitions, 269
6.8.3 Metal—metal transitions (d—d bands), 270
6.8.4 High-spin and low-spin states, 272
6.8.5 Jahn-Teller distortion, 273
6.8.6 Charge-transfer bands, 273
6.8.7 Assigning bands of transition metal complexes, 274
6.8.8 Detection of complexes and measurement of concentrations, 275
6.8.9 Spectra of compounds of elements with partly-filled f subshells
(lanthanides and actinides), 277
Mossbauer Spectroscopy, 280
7.1
UH
ee:
7.4
7.5
7.6
qed
Introduction, 280
Principles, 280
Conditions for Méssbauer spectroscopy, 281
Parameters from Mossbauer spectra, 283
7.4.1 Isomer shifts, 284 .° ~
7.4.2 Electric quadrupole interactions, 288
7.4.3 Magnetic interactions, 291
7.4.4 Time- and temperature-dependent effects, 293
Difficulties, 295
Conversion electron Mossbauer spectroscopy (CEMS), 297
Structural deductions, 297
Diffraction Methods, 304
8.1
8.2
8.3
8.4
8.5
8.6
Sar
Viii
Introduction, 304
Diffraction of electrons, neutrons and X-rays, 305
Diffraction by gases, 307
8.3.1 Principles, 307
8.3.2 Interpretation of results, 310
8.3.3 Limitations, 312
Diffraction by liquids, 315
Diffraction by single crystals, 317
8.5.1 Principles, 317
8.5.2 Interpretation of results, 322
8.5.3 Limitations, 326
Diffraction by powders, 330
Neutron diffraction, 331
CONTENTS
8.8 Study of electron density distributions, 333
8.9 Extended X-ray absorption fine structure, 336
9 Mass Spectrometry, 345
9.1 Experimental arrangements, 345
9.2 Molecular ions, 347
9.3 Fragmentation, 351
9.4 Ion reactions, 353
9.5 Thermodynamic data, 357
10 Case Histories, 361
10.1 Introduction, 361
10.2 Xenon hexafluoride, 362
10.3 Tetrahydroborates, 367
10.4 Sodium orthonitrate, Na,;NO,, 374
10.5 Ferrocene: eclipsed or staggered?, 375
10.6 Tetramethylplatinum, 378
10.7 Some Aug clusters, 380
10.8 Iron—molybdenum-—sulfur cubane-type clusters, 382
10.9 Cyclopentadienyldifluorophosphine, 385
10.10 Methylidynephosphine, HCP, 387
10.11 Electrons in reduced or excited bipyridine complexes, 388
10.12 An iridium bipyridine complex, 392
10.13 An iridium phosphine complex, 394
10.14 A boron disulfide macrocycle, 395
10.15 Structure of N,O3, 396
10.16 Fe(CO),, 397
10.17 The cation Xe,*, 400
10.18 Chromium—chromium quadruple bonds, 403
10.19 Bridging hydrogen atoms in methyl—metal compounds, 406
Appendix: Symmetry, 414
Symmetry operations, 414
Point groups, 418
Characters and symmetry species, 422
Permutation groups, 425
Point group character tables, 427
Problems — Solutions and Comments, 433
Compound Index, 441
Subject Index, 448
CONTENTS