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Contents Preface CHAPTER I. What can we do with Frontier Orbitals? I. The advantages of the perturbation method II. The uses of frontier orbitals Five standard frontier orbital treatments of reactivity Three standard frontier orbital treatments of structural problems CHAPTER II. Atomic and molecular orbitals I. Atomic orbitals II. Molecular orbitals III. The MO?s of a homonuclear diatomic molecule 1. Calculations 2. A physical interpretation A. Molecular orbitals B. The parameters The Coulomb integral a The resonance integral b The overlap integral C. Mulliken analysis Overlap population Net atomic charges 14 IV. MO?s of a heteronuclear diatomic molecule 1. Calculations 2. A physical interpretation V. p MO?s of polyatomic molecules 1. The H¿ckel method for polyatomic molecules A. The H¿ckel method applied to the allyl system B. Coulson formulae for linear polyenes C. Bond orders and net charges 2. How to calculate H¿ckel MO?s A. Choosing the parameters a end b Heteroatoms Alkyl substituents The methyl inductive effect B. Writing the secular determinant C. Checking the calculations D. Electron counting To dig deeper CHAPTER III. The perturbation method I. Perturbations and H¿ckel methods II. Study of bimolecular reactions using perturbation methods 1. Two orbital systems A. The MO?s in the starting materials are degenerate B. The MO?s in the starting materials are not degenerate Remarks 2. Systems having more than two orbitals 3. The frontier orbital approximation 4. Unimolecular systems III. Perturbation theory: the practical aspects 1. Numerical calculations 2. Qualitative applications IV. The Dewar PMO method 1. Alternant hydrocarbons The pairing theorem 34 2. The Dewar PMO method H¿ckel rule Generalized aromaticity rules Dewar-Zimmerman rule 3. Advantages and disadvantages of the PMO method A. Advantages B. Disadvantages To dig deeper CHAPTER IV. Absolute and relative reactivities I. Absolute reactivity 1. Bimolecular reactions A. Cycloadditions B. Justification of the frontier orbital approximation 2. Unimolecular reactions A. Sigmatropic reactions B. Electrocyclic reactions Limitations of the foregoing rules II. Relative reactivity 1. Electrophilic reactions Geometry of ?ate? complexes 2. Nucleophilic reactions A. The nucleophilicity of halides B. Electrophilic assistance C. An example of chemoselectivity: relative reactivities of carbonyl compounds 3. Cycloadditions Alder?s rule III. Limitations of rules 1 and 2 1. Some difficulties encountered with rule 1 A. Cheletropic reactions Electron count in cheletropic reactions B. Systems having more than two components 2. Problems with rule 2 A. Apparent violations of rule 2 The relative reactivities of alkenes and alkynes The relative reactivities of carbonyl compounds Staudinger reaction Exceptions and how to predict them B. Why overlap is ignored C. ?Formal? frontier orbitals and ?chemical? frontier orbitals CHAPTER V. Regioselectivity I. Cycloadditions II. Electrophilic reactions 1. Markovnikov?s rule 2. Regioselectivity involving enols and enolates Regioselectivity in sulfur compounds 3. FO theory and ionic reactions FO study of ionic reactions using H¿ckel calculations FO study of ionic reactions using SCF methods FO theory and gas phase reactions III. Nucleophilic reactions Additions to conjugated carbonyl compounds IV. Radical reactions V. Periselectivity VI. Limitations of rule 3 CHAPTER VI. Stereoselectivity I. Pericyclic reactions 1. Electrocyclic reactions 2. A. Torquoselectivity 3. B. Extensions of the Rondan-Houk treatment and quantitative analyses Generalizations Quantitative analyses C. An exercise in qualitative analysis 2. Sigmatropic rearrangements A. Torquoselectivity B. Cope rearrangements 3. Cycloadditions and their orientations A. Endo-exo orientation B. Syn-anti orientation II. Addition reactions 1. Nucleophilic additions 2. A. The non-perpendicular Dunitz-B¿rgi attack B. Baldwin?s cyclization rules C. 1,2 asymmetric inductions The Cram and Felkin models The flattening rule The importance of being flexible Classification of substituents as L, M, S and the Cieplak model Karabatsos model D. Which factors control asymmetric induction? E. Some recent models Houk model for electrophilic additions to alkenes Morokuma model for conjugated additions Radical additions to alkenes Evans electrostatic models for 1,2 and 1,3 asymmetric inductions The inside alkoxy effect 2. Electrophilic additions 3. Application to the aldol addition III. Substitution reactions 1. Bimolecular electrophilic substitutions 2. Bimolecular nucleophilic substitutions 3. Application to the aldol addition IV. Limitations of rule 4 CHAPTER VII. Some structural problems I. Principle of the method II. Stable conformations 1. Aldehydes, alkenes and enol ethers A. Ethanal and propene B. Propanal and methyl vinyl ether C. Chloroethanal and 2-chloropropanal 2. Conformations of some ions Walsh orbitals A. The cyclopropylcarbinyl cation B. Substituted ethyl ions The stereochemistry of vinylic SN2 reactions 3. The anomeric effect A. Lone pairs in ethers B. The anomeric effect C. Applications D. A warning 4. The geminal effect 5. The gauche effect Two adjacent lone pairs A lone pair adjacent to a polar bond Two adjacent polar bonds III. Reactive conformations IV. How to stabilize inherently unstable species Cyclobutadiene Trimethylenemethane Stable carbenes V. Bonds with abnormal lengths 1. Structural consequences of HOMO-LUMO interactions 2. Applications to nucleophilic additions A. Additions and additions-eliminations B. Reversible and irreversible reactions. Multistep mechanisms 3. Substituent effects A. Fragmentations, enolizations and related reactions Cleaving CC bonds Cleaving CH bonds B. The Cope reaction Substitutions at positions 3 and 4. The anionic oxy-Cope reaction Substitutions at other positions Polysubatitutions C. The Claisen reaction D. Substituted cyclopropanes VI. Abnormal valence angles CHAPTER VIII. Going further I. The limits of frontier orbital theory 1. The simplifying hypotheses of frontier orbital theory 2. Consequences Limitations imposed by approximation (1) Limitations imposed by approximation (2) Limitations imposed by approximation (3) Limitations imposed by approximation (4) Limitations imposed by approximation (5) II. The capabilities of computational chemistry 1. Structural problems 2. Reactivity problems 3. Beyond potential surfaces III. The methods of quantum chemistry 1. The approximations 2. 2. The principal theoretical models A. Ab initio and semi-empirical models B. Basis sets C. Correlation models Configuration interaction models Moller-Plesset models DFT models D . Solvent effects 3. A few technical points Choosing the model Choosing a basis set To dig deeper ANNEX. MO catalogue Summary table Acknowledgements INDEX

Library of Congress Subject Headings for this publication:

Molecular orbitals.

Chemistry, Physical organic.