Dipole moments of aromatic heterocycles

Abstract

This chapter is an overview of the experimental and theoretical dipole moments of aromatic heterocycles, their determination, and computation. In this chapter, the material presented illustrates the difficulties connected with the experimental determination and theoretical computation of dipole moments and heterocyclic compounds, in particular. In molecules containing atoms of different electronegativity, as is normally the case with heterocyclic compounds, the electrons are not shared equally by the respective atoms and this results in regions of high electron density and of low electron density. Because of this uneven distribution of electrons, a molecule, which as a whole is electroneutral, will possess a center of positive charge (positive end) and a center of negative charge (negative end). If these two centers do not coincide, the molecule has a permanent electric dipole moment. A dipole moment represents a direct measure of electron distribution in a molecule of known geometry. It is a physical constant that can be obtained experimentally and calculated. The electric dipole moment is a vector. The chapter defines the positive direction of the dipole moment as the direction from the center of the positive charge toward the center of the negative charge. Most heterocyclic compounds possess an uneven distribution of charges resulting in a permanent dipole moment. Typical dipole moments for most organic molecules are in the range between 0 and 12 D but there are some compounds such as polymethine dyes, which have dipole moments of 20 D or higher. The chapter describes experimental ground-state dipole moments—dielectric constant, microwave, electric resonance, and Raman spectroscopy. Sign and direction of the dipole moment has been explained. There is the explanation of calculated ground-state dipole moments—empirical, semiempirical, and ab initio methods, also a comparison of semiempirical and ab initio methods. There are details on the experimental excited-state dipole moments and the calculated excited-state dipole moments.