Similarity and complementarity in chemistry

Abstract

All properties of an atom or a functional grouping of atoms are defined by quantum mechanics. Atoms are the most transferable pieces of a system that can be defined in real space and they thus maximize the transfer of chemical information from one system to another. This is the property essential to the definition of chemical similarity, which we propose should be defined and determined by a direct comparison of atomic properties, both average and local. This definition is illustrated by an application to peptide groups. It is shown that, in limiting cases, similarity can be great enough to enable transfer of a peptide group from one system to another without change. The role of similarity in determining molecular complementarity is discussed. Molecular size or shape—van der Waals complementarity—is distinguished from Lewis complementarity, which occurs in acid-base intermolecular interactions. It is shown that the Laplacian of the electronic charge distribution can be used to predict Lewis complementarity. This definition is illustrated by a comparison of the Laplacian distributions ofcreatine and carbamoyl sarcosine, its inhibitor with respect to creatinase. The paper concludes with a discussion of how the theory of atoms can be used to provide a unified physical basis for the mathematically based models of similarity.