Functional Groups in Quantum Chemistry

Abstract

A chemical functional group is a collection of nuclei and the associated electron density, which occur with a similar nuclear arrangement and a similar local electron density cloud in many molecules. A functional group is a molecular moiety of a specific stoichiometry that typically undergoes similar chemical reactions in most molecules containing this moiety. In chemistry, especially in organic chemistry, the concept of functional groups is a powerful tool used for the characterization and classification of molecules and their reactions; in fact, the presence of a given functional group in a family of molecules is an expression of chemical similarity. Molecules containing common functional groups often exhibit similar physical properties in addition to their similar chemical properties. This chapter describes the quantum chemical concept of functional groups, following a topological approach, based on the three-dimensional shape of fuzzy molecular bodies and the local shapes of various molecular moieties. This quantum chemical description of functional groups was proposed, based on the topological shape analysis of molecules and on the density domain approach to chemical bonding. Density domains are formal bodies enclosed by molecular isodensity contours; density domains play an important role in molecular shape analysis and serve as the basis of various molecular similarity measures. The proposed quantum chemical model of functional groups also fits within a rather broad, essentially geometrical framework.