With a view to obtaining a picture for an idealized path of chemical reactions the concept of intrinsic reaction coordinate (IRC) is introduced within the space of the multidimensional potential energy function of the reacting system. The IRC uniquely determines, in a classical sense, the mode of deformation of chemically reacting molecules. On every point along the IRC an orbital analysis is possible with respect to each of the deformed molecules, the geometry of which is frozen to that in the reacting composite system. The analysis is carried out by constructing orbital pairs by respective unitary transformations of canonical molecular orbitals of the deformed molecules, so as to diagonalize the interaction matrix. Of these orbital pairs one or a few, localized in the reacting domain, play the dominant role in offering a distinct view of bond-forming processes along the IRC. In the process of these orbital analyses, the importance of electron delocalization in the chemical interaction is recognized and stressed.

Introduction

Complicated chemical phenomena, for example organic chemical reactions, are probably one of the most distant ‘lands’ from the Schrödinger equation in the vast world of its appications. In fact, the chemical reaction is the field which was forsaken by theoretical physicists. In the early stage of the development of applied quantum mechanics, many of the best theorists turned their back upon complex chemical reactions principally on account of the laboriousness involved in computations.