The Invariance of Molecular Topology in Chemical Reactions

There are two different pictures of molecular structure: the classical and the quantummechanical. The classical picture is naive-empirical and is the chemical one; it is connected with classical structural formulae, ball-and-stick models, the phenomenological Lewis concept and the Gillespy rules for prediction of molecular geometry. This picture now endures as the heuristic instrument for the planning of chemical synthesis, for communication between experimental chemists, and for chemical education. The quantum-mechanical picture is the physical one; it is based on the application of quantum mechanical ideas to molecular structures and on quantum-chemical calculations of different degrees of sophistication. Many attempts have been made in theoretical chemistry to find some symbiosis between these two different levels of description of molecular structure; only in recent years the desired compromise seems to have been found in the topological nature of both the quantum-mechanical and classical models of the molecular structure.