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Introduction
In the past four years, we published various papers with the purpose of drawing the attention of chemists to the following possibilities: a) Nonbonded interactions and their influence on torsional isomerism can be understood within the context of simple one electron MO theory 1-8). These ideas are discussed in Parts I and II. b) Sigma interactions, designated geminal interactions, may affect the shape of molecules and associated shape-related properties in a manner which is also understandable within the context of one electron MO theory 9). These concepts are presented in Part III. , c) Chemical reactivity can be understood in terms of donor-acceptor interactions with definite trends being expected as one reactant becomes an increasingly better donor and the other an increasingly better acceptor 1~ These ideas have now been applied in an explicit manner to problems of molecular structure and are discussed in Part IV under the heading of conjugative interactions. d) Bond ionicity effects can be best understood by means of an effective one electron configuration interaction approach in a way which is suitable for the formulation of general predictive rules 1s). These ideas are discussed in Part V. These sense in which terms like conjugative interactions, nonbonded interactions, etc., are meant will become clear when we discuss each individual type of interaction or effect. Suffice to say that, in many instances, conjugative interactions as well as geminal interactions or bond ionicity effects contain implicitly the idea of nonbonded interactions. Thus, it should be emphasized that the labels of the basic types of interactions proposed here reflect the way in which the problem is formulated rather than different electronic principles. In Parts I and II, a molecule.is viewed as a composite of submolecular fragments each described by delocalized MO's. Nonbonded and geminal intera