Aromaticity and Antiaromaticity
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Aromaticity and Antiaromaticity
Concepts and Applications
Krygowski, Tadeusz M.; Merino, Gabriel; Boldyrev, Alexander I.; Cyranski, Michal K.; Sola, Miquel
John Wiley & Sons Inc
10/2022
320
Dura
Inglês
9781119085898
15 a 20 dias
Descrição não disponível.
Foreword xi
Preface xiii
List of Abbreviations xvii
1 Historical Overview 1
References 7
2 Simple Electron Counting Rules 11
2.1 Introduction 11
2.2 Hueckel's 4n +2 Rule 12
2.3 Baird's 4n ?-Electron Rule for the Lowest-Lying Triplet Excited State 13
2.4 Soncini and Fowler's Rule 16
2.5 Moebius' 4n ?-Electron Rule 16
2.6 The Linking Number Rule 18
2.7 Platt's Ring Perimeter Model 19
2.8 Clar's ?-Sextet Rule 20
2.8.1 Glidewell and Lloyd's Rule 23
2.8.2 The Y-Rule 23
2.9 Hirsch's 2(n +1)2 Rule 24
2.10 The 2n2 +2n+ 1 (S = n+ 1/2) Rule 26
2.11 Wade-Mingos' 2n+ 2 Rule 26
2.11.1 Jemmis' mno Rule 27
2.11.2 Equivalence between Hueckel's andWade-Mingos' Rules 28
2.12 Other Rules 29
References 30
3 Aromaticity from Organic to Inorganic Compounds 35
3.1 Introduction 35
3.2 ?-Aromatic Inorganic Species 36
3.3 Aromaticity in Main Group Metal Compounds 42
3.4 Aromaticity in Transition Metal Compounds 44
3.5 Conclusions 49
References 49
4 Stability and Reactivity in Aromatic Compounds 55
4.1 Introduction 55
4.2 Aromaticity and Thermodynamic Stability 56
4.3 Aromaticity and Kinetic Stability 61
4.3.1 Acenes 61
4.3.2 Pericyclic Reactions 64
4.3.2.1 Diels-Alder Cycloadditions 65
4.3.2.2 [2+2+2] Cycloadditions 67
4.3.2.3 [1,7]-Sigmatropic Migrations 68
4.3.2.4 Fullerene Additions 68
References 72
5 Descriptors of Aromaticity: Geometric Criteria 77
5.1 Introduction 77
5.2 Geometry-Based Estimation of the Molecular Energy 78
5.3 Bond Length Alternation as a Basis for Defining Aromaticity Indices 80
5.3.1 The Julg Aromaticity Index AJ 81
5.3.2 The Harmonic Oscillator Model of Aromaticity, HOMA (1972 and 1993) 82
5.4 Separation of HOMA into Two Components EN and GEO (1996) 85
5.5 Harmonic Oscillator Model of Electron Delocalization, HOMED (2007) 86
5.6 Harmonic Oscillator Model for Heterocycles with ? Electron and/or n-Electron Delocalization: The HOMHED Index (2012) 87
5.7 Applications of the Bond Orders for Estimating Aromaticity 88
5.8 Bird's Aromaticity Indices I5 and I6 (1985) 90
5.9 Pozharskii Criterion of Aromaticity, ?N (1985) and Bond Alternation Coefficient, BAC (1995) 90
5.10 Applications 91
5.11 Impact of the Electric Field on Aromaticity 94
5.12 Stacking Interactions versus H-Bonding in Nucleobases 95
5.13 Showing the Interaction Path for Substituent Effects 96
5.14 Applications in the Field of Quasiaromatic Systems 100
5.15 Extension of HOMA to Noncyclic and Non-?-electron Systems 101
5.16 Conclusions 103
References 104
6 Descriptors of Aromaticity: Energetic Criteria 111
6.1 Introduction 111
6.2 Thermochemical Approaches 114
6.3 Energetic Approaches Based on Molecular Geometry 115
6.4 Theoretical Approaches 117
References 126
7 Descriptors of Aromaticity: Magnetic Criteria 131
7.1 Introduction 131
7.2 NMR Chemical Shifts 134
7.3 Nucleus Independent Chemical Shifts 135
7.4 Magnetically Induced Current Densities 138
7.5 Anisotropy of the Induced Current Density Tensor 140
References 141
8 Descriptors of Aromaticity: Electronic Criteria 145
8.1 Introduction 145
8.2 Density Functions 146
8.3 Measures of Electron Delocalization 150
8.3.1 The Electron Sharing Indices (ESI) 150
8.3.2 The Electron Localization Function (ELF) 152
8.4 Electronic Descriptors of Aromaticity 155
References 160
9 Heteroaromaticity 165
9.1 Introduction 165
9.2 Six-Membered Organic and Inorganic Heterocycles 168
9.3 Polycyclic Heteroaromatic Hydrocarbons 178
9.4 Five-Membered Organic Heterocycles 179
9.5 Aromaticity of Nucleic Bases 183
References 187
10 Moebius Aromaticity 193
10.1 Introduction 193
10.2 Metallacyclic Moebius Aromatic Species 197
10.3 Macrocyclic Moebius Aromaticity 199
References 204
11 ??-, ??-, ??-, and ??-Aromaticity 207
11.1 Introduction 207
11.2 ?-Aromatic and ?-Antiaromatic Species 209
11.2.1 ?-Aromatic Species 209
11.2.2 ?-Antiaromatic Species 210
11.3 ?-, ?-Doubly Aromatic, and ?-, ?-Doubly Antiaromatic Species and Species with ?-, ?-Conflicting Aromaticity 211
11.3.1 ?-, ?-Doubly Aromatic Species 211
11.3.2 ?-, ?-Doubly Antiaromatic Species 212
11.3.3 Species with ?-Antiaromaticity and ?-Aromaticity 213
11.3.4 Species with ?-Aromaticity and ?-Antiaromaticity 214
11.4 ?-Aromaticity 215
11.5 ?-Aromaticity 218
11.6 Conclusions 219
References 220
12 The Distortivity of ??-Electrons 223
12.1 Introduction 223
12.2 The Kekulean Distortion 224
12.3 Frequencies of the Kekule Vibrational Mode in Benzene 225
12.4 Changes in Aromaticity in the Kekulean Distortion 227
12.5 The Maximum Hardness and Minimum Polarizability Principles 228
12.6 The Distortive Nature of ?-Electrons 229
12.7 Conclusions 236
References 236
13 Three-Dimensional Aromaticity 241
13.1 Introduction 241
13.2 Spherical Aromaticity 242
13.2.1 Aromaticity on the Surface of the Sphere 242
13.2.2 Aromaticity Inside the Sphere 247
13.2.2.1 Closo Boranes 248
13.2.2.2 Jellium Cluster Model 250
13.3 Octahedral Aromaticity 251
13.4 Cubic Aromaticity 253
13.5 Tetrahedral Aromaticity 255
13.6 Cylindrical Aromaticity 256
References 259
14 Excited State Aromaticity 265
14.1 Introduction 265
14.2 Theoretical and Experimental Studies of Excited State Aromaticity 266
14.2.1 Theoretical and Computational Studies 267
14.2.2 Experimental Studies 269
14.3 Influence of Aromaticity in the Excited State Properties 271
14.3.1 Molecular Dipole Moments 271
14.3.2 Singlet-Triplet Energy Gaps 272
14.3.3 Photoacidity 273
14.4 Influence of Aromaticity in the Excited State Reactivity 274
14.4.1 Photoisomerizations 275
14.4.2 Excited State Intramolecular Proton Transfer 276
14.4.3 Photochemical Formation of Ortho-Xylylenes 278
14.4.4 Photochemical Pericyclic Reactions 279
References 281
Index 285
Preface xiii
List of Abbreviations xvii
1 Historical Overview 1
References 7
2 Simple Electron Counting Rules 11
2.1 Introduction 11
2.2 Hueckel's 4n +2 Rule 12
2.3 Baird's 4n ?-Electron Rule for the Lowest-Lying Triplet Excited State 13
2.4 Soncini and Fowler's Rule 16
2.5 Moebius' 4n ?-Electron Rule 16
2.6 The Linking Number Rule 18
2.7 Platt's Ring Perimeter Model 19
2.8 Clar's ?-Sextet Rule 20
2.8.1 Glidewell and Lloyd's Rule 23
2.8.2 The Y-Rule 23
2.9 Hirsch's 2(n +1)2 Rule 24
2.10 The 2n2 +2n+ 1 (S = n+ 1/2) Rule 26
2.11 Wade-Mingos' 2n+ 2 Rule 26
2.11.1 Jemmis' mno Rule 27
2.11.2 Equivalence between Hueckel's andWade-Mingos' Rules 28
2.12 Other Rules 29
References 30
3 Aromaticity from Organic to Inorganic Compounds 35
3.1 Introduction 35
3.2 ?-Aromatic Inorganic Species 36
3.3 Aromaticity in Main Group Metal Compounds 42
3.4 Aromaticity in Transition Metal Compounds 44
3.5 Conclusions 49
References 49
4 Stability and Reactivity in Aromatic Compounds 55
4.1 Introduction 55
4.2 Aromaticity and Thermodynamic Stability 56
4.3 Aromaticity and Kinetic Stability 61
4.3.1 Acenes 61
4.3.2 Pericyclic Reactions 64
4.3.2.1 Diels-Alder Cycloadditions 65
4.3.2.2 [2+2+2] Cycloadditions 67
4.3.2.3 [1,7]-Sigmatropic Migrations 68
4.3.2.4 Fullerene Additions 68
References 72
5 Descriptors of Aromaticity: Geometric Criteria 77
5.1 Introduction 77
5.2 Geometry-Based Estimation of the Molecular Energy 78
5.3 Bond Length Alternation as a Basis for Defining Aromaticity Indices 80
5.3.1 The Julg Aromaticity Index AJ 81
5.3.2 The Harmonic Oscillator Model of Aromaticity, HOMA (1972 and 1993) 82
5.4 Separation of HOMA into Two Components EN and GEO (1996) 85
5.5 Harmonic Oscillator Model of Electron Delocalization, HOMED (2007) 86
5.6 Harmonic Oscillator Model for Heterocycles with ? Electron and/or n-Electron Delocalization: The HOMHED Index (2012) 87
5.7 Applications of the Bond Orders for Estimating Aromaticity 88
5.8 Bird's Aromaticity Indices I5 and I6 (1985) 90
5.9 Pozharskii Criterion of Aromaticity, ?N (1985) and Bond Alternation Coefficient, BAC (1995) 90
5.10 Applications 91
5.11 Impact of the Electric Field on Aromaticity 94
5.12 Stacking Interactions versus H-Bonding in Nucleobases 95
5.13 Showing the Interaction Path for Substituent Effects 96
5.14 Applications in the Field of Quasiaromatic Systems 100
5.15 Extension of HOMA to Noncyclic and Non-?-electron Systems 101
5.16 Conclusions 103
References 104
6 Descriptors of Aromaticity: Energetic Criteria 111
6.1 Introduction 111
6.2 Thermochemical Approaches 114
6.3 Energetic Approaches Based on Molecular Geometry 115
6.4 Theoretical Approaches 117
References 126
7 Descriptors of Aromaticity: Magnetic Criteria 131
7.1 Introduction 131
7.2 NMR Chemical Shifts 134
7.3 Nucleus Independent Chemical Shifts 135
7.4 Magnetically Induced Current Densities 138
7.5 Anisotropy of the Induced Current Density Tensor 140
References 141
8 Descriptors of Aromaticity: Electronic Criteria 145
8.1 Introduction 145
8.2 Density Functions 146
8.3 Measures of Electron Delocalization 150
8.3.1 The Electron Sharing Indices (ESI) 150
8.3.2 The Electron Localization Function (ELF) 152
8.4 Electronic Descriptors of Aromaticity 155
References 160
9 Heteroaromaticity 165
9.1 Introduction 165
9.2 Six-Membered Organic and Inorganic Heterocycles 168
9.3 Polycyclic Heteroaromatic Hydrocarbons 178
9.4 Five-Membered Organic Heterocycles 179
9.5 Aromaticity of Nucleic Bases 183
References 187
10 Moebius Aromaticity 193
10.1 Introduction 193
10.2 Metallacyclic Moebius Aromatic Species 197
10.3 Macrocyclic Moebius Aromaticity 199
References 204
11 ??-, ??-, ??-, and ??-Aromaticity 207
11.1 Introduction 207
11.2 ?-Aromatic and ?-Antiaromatic Species 209
11.2.1 ?-Aromatic Species 209
11.2.2 ?-Antiaromatic Species 210
11.3 ?-, ?-Doubly Aromatic, and ?-, ?-Doubly Antiaromatic Species and Species with ?-, ?-Conflicting Aromaticity 211
11.3.1 ?-, ?-Doubly Aromatic Species 211
11.3.2 ?-, ?-Doubly Antiaromatic Species 212
11.3.3 Species with ?-Antiaromaticity and ?-Aromaticity 213
11.3.4 Species with ?-Aromaticity and ?-Antiaromaticity 214
11.4 ?-Aromaticity 215
11.5 ?-Aromaticity 218
11.6 Conclusions 219
References 220
12 The Distortivity of ??-Electrons 223
12.1 Introduction 223
12.2 The Kekulean Distortion 224
12.3 Frequencies of the Kekule Vibrational Mode in Benzene 225
12.4 Changes in Aromaticity in the Kekulean Distortion 227
12.5 The Maximum Hardness and Minimum Polarizability Principles 228
12.6 The Distortive Nature of ?-Electrons 229
12.7 Conclusions 236
References 236
13 Three-Dimensional Aromaticity 241
13.1 Introduction 241
13.2 Spherical Aromaticity 242
13.2.1 Aromaticity on the Surface of the Sphere 242
13.2.2 Aromaticity Inside the Sphere 247
13.2.2.1 Closo Boranes 248
13.2.2.2 Jellium Cluster Model 250
13.3 Octahedral Aromaticity 251
13.4 Cubic Aromaticity 253
13.5 Tetrahedral Aromaticity 255
13.6 Cylindrical Aromaticity 256
References 259
14 Excited State Aromaticity 265
14.1 Introduction 265
14.2 Theoretical and Experimental Studies of Excited State Aromaticity 266
14.2.1 Theoretical and Computational Studies 267
14.2.2 Experimental Studies 269
14.3 Influence of Aromaticity in the Excited State Properties 271
14.3.1 Molecular Dipole Moments 271
14.3.2 Singlet-Triplet Energy Gaps 272
14.3.3 Photoacidity 273
14.4 Influence of Aromaticity in the Excited State Reactivity 274
14.4.1 Photoisomerizations 275
14.4.2 Excited State Intramolecular Proton Transfer 276
14.4.3 Photochemical Formation of Ortho-Xylylenes 278
14.4.4 Photochemical Pericyclic Reactions 279
References 281
Index 285
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Aromaticity textbook; aromaticity handbook; aromaticity reference; aromaticity characteristics; aromaticity properties; chemical properties of aromatics; chemical characteristics of aromatics; characterization of aromatics; aromatics applications
Foreword xi
Preface xiii
List of Abbreviations xvii
1 Historical Overview 1
References 7
2 Simple Electron Counting Rules 11
2.1 Introduction 11
2.2 Hueckel's 4n +2 Rule 12
2.3 Baird's 4n ?-Electron Rule for the Lowest-Lying Triplet Excited State 13
2.4 Soncini and Fowler's Rule 16
2.5 Moebius' 4n ?-Electron Rule 16
2.6 The Linking Number Rule 18
2.7 Platt's Ring Perimeter Model 19
2.8 Clar's ?-Sextet Rule 20
2.8.1 Glidewell and Lloyd's Rule 23
2.8.2 The Y-Rule 23
2.9 Hirsch's 2(n +1)2 Rule 24
2.10 The 2n2 +2n+ 1 (S = n+ 1/2) Rule 26
2.11 Wade-Mingos' 2n+ 2 Rule 26
2.11.1 Jemmis' mno Rule 27
2.11.2 Equivalence between Hueckel's andWade-Mingos' Rules 28
2.12 Other Rules 29
References 30
3 Aromaticity from Organic to Inorganic Compounds 35
3.1 Introduction 35
3.2 ?-Aromatic Inorganic Species 36
3.3 Aromaticity in Main Group Metal Compounds 42
3.4 Aromaticity in Transition Metal Compounds 44
3.5 Conclusions 49
References 49
4 Stability and Reactivity in Aromatic Compounds 55
4.1 Introduction 55
4.2 Aromaticity and Thermodynamic Stability 56
4.3 Aromaticity and Kinetic Stability 61
4.3.1 Acenes 61
4.3.2 Pericyclic Reactions 64
4.3.2.1 Diels-Alder Cycloadditions 65
4.3.2.2 [2+2+2] Cycloadditions 67
4.3.2.3 [1,7]-Sigmatropic Migrations 68
4.3.2.4 Fullerene Additions 68
References 72
5 Descriptors of Aromaticity: Geometric Criteria 77
5.1 Introduction 77
5.2 Geometry-Based Estimation of the Molecular Energy 78
5.3 Bond Length Alternation as a Basis for Defining Aromaticity Indices 80
5.3.1 The Julg Aromaticity Index AJ 81
5.3.2 The Harmonic Oscillator Model of Aromaticity, HOMA (1972 and 1993) 82
5.4 Separation of HOMA into Two Components EN and GEO (1996) 85
5.5 Harmonic Oscillator Model of Electron Delocalization, HOMED (2007) 86
5.6 Harmonic Oscillator Model for Heterocycles with ? Electron and/or n-Electron Delocalization: The HOMHED Index (2012) 87
5.7 Applications of the Bond Orders for Estimating Aromaticity 88
5.8 Bird's Aromaticity Indices I5 and I6 (1985) 90
5.9 Pozharskii Criterion of Aromaticity, ?N (1985) and Bond Alternation Coefficient, BAC (1995) 90
5.10 Applications 91
5.11 Impact of the Electric Field on Aromaticity 94
5.12 Stacking Interactions versus H-Bonding in Nucleobases 95
5.13 Showing the Interaction Path for Substituent Effects 96
5.14 Applications in the Field of Quasiaromatic Systems 100
5.15 Extension of HOMA to Noncyclic and Non-?-electron Systems 101
5.16 Conclusions 103
References 104
6 Descriptors of Aromaticity: Energetic Criteria 111
6.1 Introduction 111
6.2 Thermochemical Approaches 114
6.3 Energetic Approaches Based on Molecular Geometry 115
6.4 Theoretical Approaches 117
References 126
7 Descriptors of Aromaticity: Magnetic Criteria 131
7.1 Introduction 131
7.2 NMR Chemical Shifts 134
7.3 Nucleus Independent Chemical Shifts 135
7.4 Magnetically Induced Current Densities 138
7.5 Anisotropy of the Induced Current Density Tensor 140
References 141
8 Descriptors of Aromaticity: Electronic Criteria 145
8.1 Introduction 145
8.2 Density Functions 146
8.3 Measures of Electron Delocalization 150
8.3.1 The Electron Sharing Indices (ESI) 150
8.3.2 The Electron Localization Function (ELF) 152
8.4 Electronic Descriptors of Aromaticity 155
References 160
9 Heteroaromaticity 165
9.1 Introduction 165
9.2 Six-Membered Organic and Inorganic Heterocycles 168
9.3 Polycyclic Heteroaromatic Hydrocarbons 178
9.4 Five-Membered Organic Heterocycles 179
9.5 Aromaticity of Nucleic Bases 183
References 187
10 Moebius Aromaticity 193
10.1 Introduction 193
10.2 Metallacyclic Moebius Aromatic Species 197
10.3 Macrocyclic Moebius Aromaticity 199
References 204
11 ??-, ??-, ??-, and ??-Aromaticity 207
11.1 Introduction 207
11.2 ?-Aromatic and ?-Antiaromatic Species 209
11.2.1 ?-Aromatic Species 209
11.2.2 ?-Antiaromatic Species 210
11.3 ?-, ?-Doubly Aromatic, and ?-, ?-Doubly Antiaromatic Species and Species with ?-, ?-Conflicting Aromaticity 211
11.3.1 ?-, ?-Doubly Aromatic Species 211
11.3.2 ?-, ?-Doubly Antiaromatic Species 212
11.3.3 Species with ?-Antiaromaticity and ?-Aromaticity 213
11.3.4 Species with ?-Aromaticity and ?-Antiaromaticity 214
11.4 ?-Aromaticity 215
11.5 ?-Aromaticity 218
11.6 Conclusions 219
References 220
12 The Distortivity of ??-Electrons 223
12.1 Introduction 223
12.2 The Kekulean Distortion 224
12.3 Frequencies of the Kekule Vibrational Mode in Benzene 225
12.4 Changes in Aromaticity in the Kekulean Distortion 227
12.5 The Maximum Hardness and Minimum Polarizability Principles 228
12.6 The Distortive Nature of ?-Electrons 229
12.7 Conclusions 236
References 236
13 Three-Dimensional Aromaticity 241
13.1 Introduction 241
13.2 Spherical Aromaticity 242
13.2.1 Aromaticity on the Surface of the Sphere 242
13.2.2 Aromaticity Inside the Sphere 247
13.2.2.1 Closo Boranes 248
13.2.2.2 Jellium Cluster Model 250
13.3 Octahedral Aromaticity 251
13.4 Cubic Aromaticity 253
13.5 Tetrahedral Aromaticity 255
13.6 Cylindrical Aromaticity 256
References 259
14 Excited State Aromaticity 265
14.1 Introduction 265
14.2 Theoretical and Experimental Studies of Excited State Aromaticity 266
14.2.1 Theoretical and Computational Studies 267
14.2.2 Experimental Studies 269
14.3 Influence of Aromaticity in the Excited State Properties 271
14.3.1 Molecular Dipole Moments 271
14.3.2 Singlet-Triplet Energy Gaps 272
14.3.3 Photoacidity 273
14.4 Influence of Aromaticity in the Excited State Reactivity 274
14.4.1 Photoisomerizations 275
14.4.2 Excited State Intramolecular Proton Transfer 276
14.4.3 Photochemical Formation of Ortho-Xylylenes 278
14.4.4 Photochemical Pericyclic Reactions 279
References 281
Index 285
Preface xiii
List of Abbreviations xvii
1 Historical Overview 1
References 7
2 Simple Electron Counting Rules 11
2.1 Introduction 11
2.2 Hueckel's 4n +2 Rule 12
2.3 Baird's 4n ?-Electron Rule for the Lowest-Lying Triplet Excited State 13
2.4 Soncini and Fowler's Rule 16
2.5 Moebius' 4n ?-Electron Rule 16
2.6 The Linking Number Rule 18
2.7 Platt's Ring Perimeter Model 19
2.8 Clar's ?-Sextet Rule 20
2.8.1 Glidewell and Lloyd's Rule 23
2.8.2 The Y-Rule 23
2.9 Hirsch's 2(n +1)2 Rule 24
2.10 The 2n2 +2n+ 1 (S = n+ 1/2) Rule 26
2.11 Wade-Mingos' 2n+ 2 Rule 26
2.11.1 Jemmis' mno Rule 27
2.11.2 Equivalence between Hueckel's andWade-Mingos' Rules 28
2.12 Other Rules 29
References 30
3 Aromaticity from Organic to Inorganic Compounds 35
3.1 Introduction 35
3.2 ?-Aromatic Inorganic Species 36
3.3 Aromaticity in Main Group Metal Compounds 42
3.4 Aromaticity in Transition Metal Compounds 44
3.5 Conclusions 49
References 49
4 Stability and Reactivity in Aromatic Compounds 55
4.1 Introduction 55
4.2 Aromaticity and Thermodynamic Stability 56
4.3 Aromaticity and Kinetic Stability 61
4.3.1 Acenes 61
4.3.2 Pericyclic Reactions 64
4.3.2.1 Diels-Alder Cycloadditions 65
4.3.2.2 [2+2+2] Cycloadditions 67
4.3.2.3 [1,7]-Sigmatropic Migrations 68
4.3.2.4 Fullerene Additions 68
References 72
5 Descriptors of Aromaticity: Geometric Criteria 77
5.1 Introduction 77
5.2 Geometry-Based Estimation of the Molecular Energy 78
5.3 Bond Length Alternation as a Basis for Defining Aromaticity Indices 80
5.3.1 The Julg Aromaticity Index AJ 81
5.3.2 The Harmonic Oscillator Model of Aromaticity, HOMA (1972 and 1993) 82
5.4 Separation of HOMA into Two Components EN and GEO (1996) 85
5.5 Harmonic Oscillator Model of Electron Delocalization, HOMED (2007) 86
5.6 Harmonic Oscillator Model for Heterocycles with ? Electron and/or n-Electron Delocalization: The HOMHED Index (2012) 87
5.7 Applications of the Bond Orders for Estimating Aromaticity 88
5.8 Bird's Aromaticity Indices I5 and I6 (1985) 90
5.9 Pozharskii Criterion of Aromaticity, ?N (1985) and Bond Alternation Coefficient, BAC (1995) 90
5.10 Applications 91
5.11 Impact of the Electric Field on Aromaticity 94
5.12 Stacking Interactions versus H-Bonding in Nucleobases 95
5.13 Showing the Interaction Path for Substituent Effects 96
5.14 Applications in the Field of Quasiaromatic Systems 100
5.15 Extension of HOMA to Noncyclic and Non-?-electron Systems 101
5.16 Conclusions 103
References 104
6 Descriptors of Aromaticity: Energetic Criteria 111
6.1 Introduction 111
6.2 Thermochemical Approaches 114
6.3 Energetic Approaches Based on Molecular Geometry 115
6.4 Theoretical Approaches 117
References 126
7 Descriptors of Aromaticity: Magnetic Criteria 131
7.1 Introduction 131
7.2 NMR Chemical Shifts 134
7.3 Nucleus Independent Chemical Shifts 135
7.4 Magnetically Induced Current Densities 138
7.5 Anisotropy of the Induced Current Density Tensor 140
References 141
8 Descriptors of Aromaticity: Electronic Criteria 145
8.1 Introduction 145
8.2 Density Functions 146
8.3 Measures of Electron Delocalization 150
8.3.1 The Electron Sharing Indices (ESI) 150
8.3.2 The Electron Localization Function (ELF) 152
8.4 Electronic Descriptors of Aromaticity 155
References 160
9 Heteroaromaticity 165
9.1 Introduction 165
9.2 Six-Membered Organic and Inorganic Heterocycles 168
9.3 Polycyclic Heteroaromatic Hydrocarbons 178
9.4 Five-Membered Organic Heterocycles 179
9.5 Aromaticity of Nucleic Bases 183
References 187
10 Moebius Aromaticity 193
10.1 Introduction 193
10.2 Metallacyclic Moebius Aromatic Species 197
10.3 Macrocyclic Moebius Aromaticity 199
References 204
11 ??-, ??-, ??-, and ??-Aromaticity 207
11.1 Introduction 207
11.2 ?-Aromatic and ?-Antiaromatic Species 209
11.2.1 ?-Aromatic Species 209
11.2.2 ?-Antiaromatic Species 210
11.3 ?-, ?-Doubly Aromatic, and ?-, ?-Doubly Antiaromatic Species and Species with ?-, ?-Conflicting Aromaticity 211
11.3.1 ?-, ?-Doubly Aromatic Species 211
11.3.2 ?-, ?-Doubly Antiaromatic Species 212
11.3.3 Species with ?-Antiaromaticity and ?-Aromaticity 213
11.3.4 Species with ?-Aromaticity and ?-Antiaromaticity 214
11.4 ?-Aromaticity 215
11.5 ?-Aromaticity 218
11.6 Conclusions 219
References 220
12 The Distortivity of ??-Electrons 223
12.1 Introduction 223
12.2 The Kekulean Distortion 224
12.3 Frequencies of the Kekule Vibrational Mode in Benzene 225
12.4 Changes in Aromaticity in the Kekulean Distortion 227
12.5 The Maximum Hardness and Minimum Polarizability Principles 228
12.6 The Distortive Nature of ?-Electrons 229
12.7 Conclusions 236
References 236
13 Three-Dimensional Aromaticity 241
13.1 Introduction 241
13.2 Spherical Aromaticity 242
13.2.1 Aromaticity on the Surface of the Sphere 242
13.2.2 Aromaticity Inside the Sphere 247
13.2.2.1 Closo Boranes 248
13.2.2.2 Jellium Cluster Model 250
13.3 Octahedral Aromaticity 251
13.4 Cubic Aromaticity 253
13.5 Tetrahedral Aromaticity 255
13.6 Cylindrical Aromaticity 256
References 259
14 Excited State Aromaticity 265
14.1 Introduction 265
14.2 Theoretical and Experimental Studies of Excited State Aromaticity 266
14.2.1 Theoretical and Computational Studies 267
14.2.2 Experimental Studies 269
14.3 Influence of Aromaticity in the Excited State Properties 271
14.3.1 Molecular Dipole Moments 271
14.3.2 Singlet-Triplet Energy Gaps 272
14.3.3 Photoacidity 273
14.4 Influence of Aromaticity in the Excited State Reactivity 274
14.4.1 Photoisomerizations 275
14.4.2 Excited State Intramolecular Proton Transfer 276
14.4.3 Photochemical Formation of Ortho-Xylylenes 278
14.4.4 Photochemical Pericyclic Reactions 279
References 281
Index 285
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