The molecular interaction energy, without considering the nuclear configuration change, is discussed by partitioning it into the Coulomb, exchange, delocalization, and polarization terms. A succinct approximate expression for each term is derived, and its magnitude is discussed.The expressions obtained as the second-order perturbation terms for the delocalization and the polarization energies are in accord with those of the well-established reactivity indices, the delocalizability , and the self-atom polarizability respectively. The chemical reactivity can thus be measured by the combined sum of these four terms.In usual ionic reactions the Coulomb and the delocalization terms are more important than the exchange and the polarization terms respectively.

The reaction path of radical-radical recombination and disproportionation has been studied by means of a perturbational method with a clear orbital interaction concept, and the mechanism of these reactions has been elucidated theoretically. It has been confirmed that these termination reactions do not have a common transition state. The recombination is found to take the path which gives the maximum interaction between the singly occupied molecular orbitals (SOMO’s) of two radical species, whereas the disproportionation is shown to take the route which gives the significant charge transfer interaction from the particular doubly occupied (DO) MO of one radical to the SOMO of the other radical. The DOMO localized at the C–H bond donates electrons to the SOMO to cause the hydrogen abstraction. In the process of disproportionation, one radical which abstracts a hydrogen atom acts as an electron-acceptor and the other radical with the hydrogen atom to be abstracted as an electron-donor. The difference between the mechanism of the recombination and that of the disproportionation is clarified in terms of the mode of the orbital interaction.

A general stereoselection rule governing the electrocyclic interactions has been proposed with a molecular orbital rationale. The reactions discussed from a unified point of view cover the ring-closure of polyenes, the Diels-Alder additions, 1,4-dipolar additions, the Cope and the Claisen rearrangements, the hydrogen and the proton migrations, 1,3-dipolar additions, the ring-opening of cyclic olefins, the deamination of cyclic unsaturated imines, 1,2-noncycloadditions, 1,2-eliminations, the nucleophilic displacement with allylic rearrangements, and other cyclic intermediate or complex formations. The relation to the molecular orbital symmetry has been also discussed.The molecular orbital (MO) theory, which has early proved itself suitable for interpreting the chemical reactivity, is exhibiting its usefulness also in the problem of the stereoselectivity. In particular, the symmetry property of MO has been intimately connected to the steric course of chemical reactions. One of the present authors first pointed out the importance of the symmetry of the highest occupied (HO) MO of dienes and the lowest vacant (LV) MO of dienophiles for the occurrence of the Diels-Alder reaction.1) The MO symmetry was correlated with the stereoselectivity in the ringclosure of linear polyenes and in the ring-opening of cyclic olefins first by Woodward and Hoffmann,2) and later by Longuet-Higgins and Abrahamson,3) and by Fukui.4) Woodward and Hoffmann2) considered the symmetry of HO only, while Fukui1,4) HO and LV for discussing the interaction of two systems.These particular orbitals were coincident with those which played an important role in the MO

A simple molecular orbital theoretical treatment of the sigma-pi interaction caused by chemical reactions of planar-conjugated systems has been developed. The problem of the streoselection of the two alternative directions of configuration altering due to the hybridization change at the centers of interaction has been considered by means of a simple perturbation treatment. Two theoretical indices, which represent the conjugation stabilization by way of the sigma-pi and reagent-pi interactions, have been introduced for this purpose. The theoretical results are consistent with the results of experience so far accumulated in various organic reactions, such as additions, eliminations, substitutions, and rearrangements, of conjugated molecules.The electronic wave function for planarconjugated molecules is divided into the so called sigma and pi parts. The former is composed of the molecular orbitals (MO) which are symmetric with respect to the molecular plane and have the direction of extension on that plane, while the latter consists of the MO's antisymmetric with respect to that plane, to which their direction of maximum extension is perpendicular. The condition of this separability has been thoroughly discussed by Lykos and Parr