The PMO Theory of Organic Chemistry

This textbook introduces the perturbation molecular orbital (PMO) th,eory of organic chemistry. Organic chemistry encompasses the largest body offactual information of any of the major divisions of science. The sheer bulk of the subject matter makes many demands on any theory that attempts to systematize it. Time has shown that the PMO method meets these demands admirably. The PMO method can provide practicing chemists with both a pictorial description of bonding and qualitative theoretical results that are well founded in more sophisticated treatments. The only requirements for use of the theory are high school algebra and a pencil and paper. The treatment described in this book is by no means new. Indeed, it was developed as a complete theory of organic chemistry more than twenty years ago. Although it was demonstrably superior to resonance theory and no more complicated to use, it escaped notice for two very simple reasons. First, the original papers describing it were very condensed, perhaps even obscure, and contained few if any examples. Second, for various reasons, no general account appeared in book form until 1969,* and this was still relatively inaccessible, being in the form of a monograph where molecular orbital (MO) theory was treated mainly at a much more sophisticated level. The generality of the PMO method is illustrated by the fact that all the new developments over the last two decades can be accommodated in it.

1. Introduction to MO Theory.- 1.1. The Hydrogen Atom; Orbits and Orbitals.- 1.2. The Orbital Approximation; Helium.- 1.3. Lithium; the Pauli Principle.- 1.4. The Atoms Be-Ne; Hund's Rule.- 1.5. The Hydrogen Molecule; Molecular Orbitals.- 1.6. The Born-Oppenheimer Approximation.- 1.7. HHe+, HHe, and HLi; Perturbation Theory.- 1.8. Methane; Symmetry Orbitals.- 1.9. Photoelectron Spectroscopy and Ionization Potentials.- 1.10. Methane, Continued; Hybridization and Localized Bonds.- 1.11. Diatomic Molecules.- 1.12. The Paraffins; Localized Bonds.- 1.13. Ethylene; ? Bonds.- 1.14. Acetylene.- 1.15. Breakdown of the Localized Bond Model: Three-Center Bonds, Conjugated Molecules, and Reaction Intermediates..- 1.16. Relationships between Different Types of Delocalized Systems.- 1.17. Summary.- Problems.- Selected Reading.- References.- 2. Perturbation Theory.- 2.1. The Usefulness of Perturbation Theory.- 2.2. Types of Perturbations Involved in the Comparison of Conjugated Systems.- 2.3. Monocentric Perturbations.- 2.4. Intramolecular Union.- 2.5. Intermolecular Union.- 2.6. Multiple Union; Additivity of Perturbations.- Problems.- Selected Reading.- Reference.- 3. PMO Treatment of Conjugated Systems.- 3.1. Principles of the PMO Method; Alternant and Nonalternant Systems.- 3.2. The Pairing Theorem.- 3.3. Calculation of NBMO Coefficients.- 3.4. Distribution of Formal Charges in AHs.- 3.5. Monocentric Perturbations; Correlation of Isoconjugate Systems.- 3.6. Intermolecular Union of Even AHs.- 3.7. Multiple Union of Even AHs.- 3.8. Union of Odd AHs.- 3.9. Alternation of Bonds in Polyenes.- 3.10. Even Monocyclic Polyenes; Aromaticity and Antiaromaticity; H ückel's Rule.- 3.11. Bond Alternation in Annulenes.- 3.12. Polycyclic Polyenes.- 3.13. Intramolecular Union; Monocyclic Nonalternant Hydrocarbons.- 3.14. Essential Single and Double Bonds; General Rules for Aromaticity.- 3.15. Significance of Classical Valence Structures.- 3.16. Union of an Odd AH with an Even AH.- 3.17. H ückeland Anti-H ückel Systems.- 3.18. Effect of Heteroatoms.- 3.19. Polarization of ? Electrons.- 3.20. Stereochemistry of Nitrogen.- 3.21. Resonance Theory in the Light of the PMO Method.- Appendix, ? Energy of Union of Even AHs.- Problems.- Selected Reading.- 4. Chemical Equilibrium.- 4.1. Basic Principles.- 4.2. Factors Contributing to the Energy of Reaction.- 4.3. Reaction of AHs.- 4.4. Electron Transfer Processes; Redox Potentials.- 4.5. Nonalternant Systems.- 4.6. Effect of Heteroatoms.- 4.7. The ?-Inductive Effect.- 4.8. Classification of Substituents.- 4.9. Inductive (I) Substituents.- 4.10. Electromeric Substituents; ±E Substituents.- 4.11.+E Substituents.- 4.12. -E Substituents.- 4.13. Summary of Substituent Effects.- 4.14. Cross-Conjugation.- 4.15. Mutual Conjugation.- 4.16. The Field Effect.- 4.17. The Hammett Equation.- Problems.- Selected Reading.- References.- 5. Chemical Reactivity.- 5.1. Basic Principles.- 5.2. The Transition State Theory.- 5.3. Transition States for Aliphatic Substitution.- 5.4. Reaction Paths and Reaction Coordinates.- 5.5. The Bell-Evans-Polanyi (BEP) Principle; Relationships between Rates of Reactions and Corresponding Equilibrium Constants.- 5.6. Reactions Where Intermediates Are Involved.- 5.7. Solvent Effects; Electrostatic Interactions and the Hellman-Feynman Theorem.- 5.8. Limitations of the BEP Principle.- 5.9. Classification of Reactions.- 5.10. Prototropic Reactions of ISOB Type.- 5.11. Prototropic Reactions of EOB Type.- 5.12. Nucleophilic Aliphatic Substitution.- A. SN1 Reactions.- B. SN2 Reactions.- 5.13. Nucleophilicity and Basicity.- 5.14. Electrophilic Aliphatic Substitution.- 5.15. Radical Substitution Reactions (EOB ).- 5.16. Elimination Reactions.- 5.17. ?-Complex Reactions (E?B ).- 5.18. Electrophilic Addition (E?B and EOB ).- 5.19. ? Complexes vs. Three-Membered Rings.- 5.20. Nucleophilic Addition and Related Reactions (EOB ).- 5.21. Radical Addition and Polymerization (EOB ).- 5.22. Aromatic Substitution in Even Systems (EOB ).- 5.23. Substitution vs. Addition.- 5.24. Neighboring Group Participation.- 5.25. Some OE Reactions.- 5.26. Thermal Pericyclic Reactions (EEA and OOA ).- 5.27. Examples of Pericyclic Reactions.- A. Cycloaddition Reactions (EEA and OOA ).- B. Some Special Features of the Diels-Alder Reaction.- C. Sigmatropic Reactions (EEA and OOA ).- D. Electrocyclic Reactions.- E. Chelotropic Reactions.- 5.28. Alternative Derivations of the Woodward-Hoffman Rules. "Allowed" and "Forbidden" Pericyclic Reactions.- 5.29. Catalysis of Pericyclic Reactions by Transition Metals.- 5.30. Reactions Involving Biradical Intermediates (ERA ).- 5.31. The ±E Substituent Technique.- Problems.- Selected Reading.- References.- 6. Light Absorption and Photochemistry.- 6.1. Introduction.- 6.2. The Nature of Electronically Excited States.- 6.3. The Franck-Condon Principle.- 6.4. Singlet and Triplet States.- 6.5. Extinction Coefficients and Transition Moments.- 6.6. Excitation and Deexcitation; Lifetimes of States, Fluorescence, and Phosphorescence.- 6.7. Excitation Energies of Even AHs.- 6.8. Excitation Energies of Odd AHs.- 6.9. ?? ?* and n ? ?* in Even, Heteroconjugated Systems.- 6.10. ?? ?* Transitions in Odd, Heteroconjugated Systems.- 6.11. Effect of Substituents on Light Absorption.- 6.12. Basic Principles of Photochemistry; Types of Photochemical Process.- 6.13. The Role of the Born-Oppenheimer (BO) Approximation.- 6.14. The Role of Antibonding Electrons.- 6.15. Classification of Photochemical Reactions.- 6.16. Examples of Photochemical Reactions.- A. X-Type Reactions.- B. GR Reactions.- C. GJ Reactions.- D. GN Reactions.- E. GA Reactions.- 6.17. Chemiluminescent Reactions.- 6.18. Summary.- Problems.- Selected Reading.- References.- 7. Reactions of Transient Ions.- 7.1. Ions in the Gas Phase; the Mass Spectrometer and Ion Cyclotron Spectroscopy.- 7.2. The Structure of Radical Ions.- 7.3. Reaction of Cation Radicals.- A. Cleavage Reactions.- B. Internal Displacement Reactions.- C. Pericyclic Reactions.- 7.4. Radical Anions in the Gas Phase.- 7.5. Ion-Molecule Reactions in the Gas Phase.- 7.6. Radical Cations in Solution.- A. Oxidation by Electron Transfer.- B. Electrochemical (Anodic) Oxidation.- C. Photochemical and Radiochemical Oxidation.- 7.7. Radical Anions in Solution.- Problems.- Selected Reading.- References.- Answers to Selected Problems.