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 principle of the stereoselection in organic chemistry has been discussed from the point of view of the molecular orbital theory. The stereoselectivity in the stereospecific ring-closure of conjugated polyenes and the stereospecific ring-cleavage of unsaturated carbon cycles, trans-1, 2-noncycloadditions, cis-1, 2-additions and cis-1, 3-dipolar additions have been accounted for with a qualitative generalization. In many cases the symmetry relationship of certain particular molecular orbitals ("frontier orbitals") plays a conspicuously significant role inthe steric control of stereochemical processes. This may be put in contrast with the role of the frontier electron density in the orientation process in organic reactions.The purpose of the present paper is to make an attempt to furnish organic stereochemistry with a theoretical strategy. In view of the multifariousness noticed in the accumulation of recent stereochemical studies, it is hoped that a guiding principle to unravel the entanglements involved will appear. In order to be of practical applicability, such a guiding principle must first of all possess generality and simplicity. Therefore, the molecular orbital (MO) theory seems suitable.The recent experimental information on the stereoselectivity in several cyclic reactions of unsaturated compounds1) has been an incentive to start an MO-theoretical attack.2-4) All these papers pointed out the importance of the symmetry of particular MO's in discussing the problem in question. These particular MO's were the highest-occupied MO (HO) and the lowest vacant MO (LV) of the ground state molecule. Oosterhoff2) has suggested that the difference in stereochemical behavior between thermal and photo-induced reactions may be attributed to the symmetry characteristics of HO and LV. The important role of HO and LV (the frontier orbitals) in the course of the chemical interaction of organic compounds was early well established in the frontier electron theory,5,6) and the symmetry of the frontier orbitals was also duly employed for
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.
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