E-Book Content
Chem. Rev. 2001, 101, 1385−1419 1385 Structural Aspects of Aromaticity Tadeusz Marek Krygowski* and Michał Ksawery Cyran´ski Department of Chemistry, University of Warsaw, Pasteura 1, 02 093 Warsaw, Poland Received August 3, 2000 Contents I. Introduction: Importance of a Notion in the Chemical Vocabulary and an Outline of the Criteria of Aromaticity II. Geometry-Based Indices of Aromaticity A. Sources and Reliability of the Molecular Geometry B. Outline of the Aromaticity Indices Based on Bond Length Alternation C. Harmonic Oscillator Model of Aromaticity (HOMA) D. Geometry-Based Energetic Indices of Aromaticity 1. Energetics of π-Electron Hydrocarbons Evaluated from the Exponential Relationship between Bond Length and Bond Energy 2. Harmonic Oscillator Stabilization Energy: Estimation of Canonical Structure Weights and Stabilization Energy from Bond Lengths E. Discussion of the Pros and Cons of Molecular Geometry-Based Indices III. Applications A. Topological Problems B. Benzenoid Hydrocarbons C. Aza-Analogues of Benzenoid Hydrocarbons D. Other Heterocyclic Analogues of Benzenoid Hydrocarbons. E. Nonbenzenoid Alternant Hydrocarbons and Their Heteroanalogues F. Nonalternant π-Electron Hydrocarbons G. Cyclopentadienyl and Cyclopentadiene Moieties Embedded in Various Chemical Environments H. Heterocyclic Analogues of Nonalternant Hydrocarbons I. Dependence of Aromaticity on Intermolecular Interactions J. Aromaticity vs Deviation from Planarity K. Experimental Arguments for the Hiberty−Shaik Hypothesis of the Role of σ-Electron Structure in π-Electron Systems L. Substituent Effects on Aromaticity 1. Bond Localization Due to the Anneleation of Small Rings to Benzene 2. Angular Group-Induced Bond Alternation (AGIBA) 3. Charged Substituents in Benzenoid Systems IV. Conclusions 1385 1388 1388 1389 1390 1391 1391 1392 1393 1397 1398 1398 1402 1404 1405