Aromaticity and Antiaromaticity

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 Hückel’s 4n +2 Rule 12 2.3 Baird’s 4n π-Electron Rule for the Lowest-Lying Triplet Excited State 13 2.4 Soncini and Fowler’s Rule 16 2.5 Möbius’ 4n π-Electron Rule 16 2.6 The Linking Number Rule 18 2.7 Platt’s Ring Perimeter Model 19 2.8 Clar’s π-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 Hückel’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 π-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 π 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, ΔN (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-π-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 Möbius Aromaticity 193 10.1 Introduction 193 10.2 Metallacyclic Möbius Aromatic Species 197 10.3 Macrocyclic Möbius Aromaticity 199 References 204 11 𝛔-, 𝛑-, 𝛅-, and 𝛗-Aromaticity 207 11.1 Introduction 207 11.2 σ-Aromatic and σ-Antiaromatic Species 209 11.2.1 σ-Aromatic Species 209 11.2.2 σ-Antiaromatic Species 210 11.3 σ-, π-Doubly Aromatic, and σ-, π-Doubly Antiaromatic Species and Species with σ-, π-Conflicting Aromaticity 211 11.3.1 σ-, π-Doubly Aromatic Species 211 11.3.2 σ-, π-Doubly Antiaromatic Species 212 11.3.3 Species with σ-Antiaromaticity and π-Aromaticity 213 11.3.4 Species with σ-Aromaticity and π-Antiaromaticity 214 11.4 δ-Aromaticity 215 11.5 ϕ-Aromaticity 218 11.6 Conclusions 219 References 220 12 The Distortivity of 𝛑-Electrons 223 12.1 Introduction 223 12.2 The Kekulean Distortion 224 12.3 Frequencies of the Kekulé 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 π-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