Our researches, supported by the National Natural Science Foundatino of China, on the fundamental theories of organic chemistry are reviewed. A new method for ab initio energy partition is being developed in order to gain new insight into the nature of electron delocalization, and it is able to provide any molecule with a fragment MO basis set in which the π and σ systems have been separated out no matter whether molecule is planar or nonplanar as well as whether the π system is conjugated or cumulated. Contrary to the HMO theory, the π electron delocalization has a great influence upon the original π systems themselves as well as upon the σ framework due to the π-σ space interaction. Accordingly, aromatic compounds can be divided into three groups and a delocalized π system itself is always destabilized, the dependence of aromatic ring current upon the CT was reveled, and ring rigidity has been defined as a new aromatic index. Based on the distinction between the CT and EX delocalizations, it was found that the stabilizing CT interaction is practically destabilization and the destabilizing EX interaction is stabilization as far as their effects on the behaviors of whole molecule are concemed. Consequently, the conclusions that the π-π, π-σ and non-bonded σ-σ interaction are always destabilization in the case of stilbene-like species are reasonable. Out of the classical expectation, the π-π interaction has only a slight effect on molecular behaviors. Particularly, it is the σ-σ destabilization interaction, rather than nuclear repulsion as classicaly thought, to distort molecule away from its planar geometry, and stilbene-like species may exist preferentially in a "crowded" geometry. The roles of the π and σ electrons in determining molecular behaviors should be reevaluated.

Key words: ENERGY DECOMPOSITION METHOD, ACTIVE FORCE, CONJUGATION, MOLECULAR ORBIT, AROMATICITY, CONFORMATION, PHYSICAL ORGANIC CHEMISTRY