The Huckel theory, with an extended basis set consisting of 2s and 2p carbon and 1s hydrogen orbitals, with inclusion of overlap and all interactions, yields a good qualitative solution of most hydrocarbon conformational problems. Calculations have been performed within the same parametrization for nearly all simple saturated and unsaturated compounds, testing a variety of geometries for each. Barriers to internal rotation, ring conformations, and geometrical isomerism are among the topics treated. Consistent σ and π charge distributions and overlap populations are obtained for aromatics and their relative roles discussed. For alkanes and alkenes charge distributions are also presented. Failures include overemphasis on steric factors, which leads to some incorrect isomerization energies; also the failure to predict strain energies. It is stressed that the geometry of a molecule appears to be its most predictable quality.

An Extended Hückel Theory. I. Hydrocarbons

I. Introduction: Conceptual vs Fundamental andComputational Aspects of DFT1793II. Fundamental and Computational Aspects of DFT 1795A. The Basics of DFT: The Hohenberg−KohnTheorems1795B. DFT as a Tool for Calculating Atomic andMolecular Properties: The Kohn−ShamEquations1796C. Electronic Chemical Potential andElectronegativity: Bridging Computational andConceptual DFT1797III. DFT-Based Concepts and Principles 1798A. General Scheme: Nalewajski’s ChargeSensitivity Analysis1798B. Concepts and Their Calculation 18001. Electronegativity and the ElectronicChemical Potential18002. Global Hardness and Softness 18023. The Electronic Fukui Function, LocalSoftness, and Softness Kernel18074. Local Hardness and Hardness Kernel 18135. The Molecular Shape FunctionsSimilarity 18146. The Nuclear Fukui Function and ItsDerivatives18167. Spin-Polarized Generalizations 18198. Solvent Effects 18209. Time Evolution of Reactivity Indices 1821C. Principles 18221. Sanderson’s Electronegativity EqualizationPrinciple18222. Pearson’s Hard and Soft Acids andBases Principle18253. The Maximum Hardness Principle 1829IV. Applications 1833A. Atoms and Functional Groups 1833B. Molecular Properties 18381. Dipole Moment, Hardness, Softness, andRelated Properties18382. Conformation 18403. Aromaticity 1840C. Reactivity 18421. Introduction 18422. Comparison of Intramolecular ReactivitySequences18443. Comparison of Intermolecular ReactivitySequences18494. Excited States 1857D. Clusters and Catalysis 1858V. Conclusions 1860VI. Glossary of Most Important Symbols andAcronyms1860VII. Acknowledgments 1861VIII. Note Added in Proof 1862IX. References 1865

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Conceptual density functional theory.

An ab initio gauge-invariant molecular orbital theory is developed for nuclear magnetic shielding. The molecular orbitals are written as linear combinations of gauge-invariant atomic orbitals, the wavefunctions in the presence of a uniform external magnetic field being determined by self-consistent field perturbation theory. The final magnetic shielding result is broken up into contributions which can be related to various features of electronic structure. Calculated magnetic shielding constants are presented using three sets of atomic orbitals, all of which are taken as contracted gaussian-type functions. The first two sets are minimal and the third is slightly extended. All three levels of theory give good descriptions of shielding at first row and hydrogen atoms. Carbon and hydrogen chemical shifts calculated at the extended level are in excellent agreement with experimental values.

Self-consistent perturbation theory of diamagnetism

The Conservation of Orbital Symmetry

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Aryl-aryl bond formation one century after the discovery of the Ullmann reaction.

In the search for a quantitative correlation between reactivity and electronic configuration of aromatic hydrocarbons, the electron density, at each carbon atom, of the highest occupied π‐orbital in the ground state of the molecule is calculated by means of the LCAO method. Comparing the result of such a calculation on fifteen condensed aromatic hydrocarbons with their chemical reactivities, we find that the position at which the electron density is largest is most readily attacked by electrophilic or oxidizing reagents.It is, therefore, concluded that distinct from other π‐electrons the pair of π‐electrons occupying the highest orbital, which is referred to as frontier electrons, plays a decisive role in chemical activation of these hydrocarbon molecules. The theoretical significance of this discrimination of the frontier electrons in relation to the chemical activation is discussed.

A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons

As to the LCAO MO treatment of the orientation problem in chemical reactions of π‐electron systems, the frontier electron concept which has been previously introduced by the authors for explaining the reactivities in aromatic hydrocarbons is subjected to an extension in the sense that the frontier orbitals are specified according to the type of reaction. Thus, fundamental postulates relating to the reactivities of π‐electron systems are set up, which are believed to include general principles involved in the mechanism of both substitution and addition of electrophilic, nucleophilic, or radical type. On the basis of these postulates it is possible to predict the position of attack in conjugated molecules in all the three types of substitution as well as addition in a consistent manner. There is a nearly perfect agreement between the theoretical conclusions and experimental results hitherto reported.

Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic, and Other Conjugated Molecules

A theory concerning the mechanism of substitution in conjugated molecules is stated. The energy of the whole system is divided into σ- and π-parts. When certain relations hold between some integrals of σ-electrons, the σ-part of the energy of the system has a maximum in the process of the reaction, by which the transition state is defined. In the transition state a hyperconjugation may take place between the initial π-electron system and the pseudo-π-orbital in the vicinity of the reaction center. By means of the perturbation method, the π-part of the energy of the system involving the pseudo-π-orbital is obtained. The perturbation energy is always negative and by the magnitude of its absolute value, the reactivity of conjugated molecules can be discussed in a definite type of reaction with a definite reagent. From an approximate expression of this perturbation energy, the frontier electron theory is derived, which has previously been proposed by the present authors, whose coincidence with chemical experi...

Theory of Substitution in Conjugated Molecules

The effects of solvents on the g-value of di-t-butyl nitric oxide (DTBNO) have been examined. Plots of the g-value vs. the hyperfine coupling constant of 14N of DTBNO in various solvents formed two...

Solvent Effects on the g-Value of Di-t-butyl Nitric Oxide

An MO‐theoretical investigation on the mechanism of aromatic substitution is made. Supposing that the approaching reagent and the atom to be replaced can be treated as an atom (pseudoatom) which has a π‐type orbital in the course of reaction, an important role of hyperconjugation taking place between the pseudoatom and the aromatic compound is remarked. Since the electron density at the pseudoatom relates to the amount of charge transfer through this hyperconjugation and varies as the reaction proceeds, the proceeding of the reaction can be measured by this electron density. Then, the electron density is expressed, in the form of a contour diagram, in terms of the Coulomb integral of the pseudoatom and the resonance integral between the pseudoatom and carbon atom to be attacked. Thus, a reaction path can be represented by a locus plotted on this diagram. The discriminating property of frontier orbitals can be observed in this diagram. It has been confirmed that an electrophilic, or a nucleophilic, substitution takes place only when a certain condition is satisfied with respect to the energy of the reagent. By determining the transition state which corresponds to the end point of the locus, several reactivity indices, i.e., frontier electron density, superdelocalizability, its one‐term approximation and a generalized reactivity index are derived from the hyperconjugation energy at the transition state, which is also available as a generalized reactivity index. Correlation between these and the existing indices is examined.

Teijiro Yonezawa

Papers

A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons

Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic, and Other Conjugated Molecules

Theory of Substitution in Conjugated Molecules

Solvent Effects on the g-Value of Di-t-butyl Nitric Oxide

MO-Theoretical Approach to the Mechanism of Charge Transfer in the Process of Aromatic Substitutions