Charge calculations in molecular mechanics IV: A general method for conjugated systems

Abstract

Abstract A general method for calculating pi charges based on the Hückel molecular orbital (HMO) approach is incorporated into an existing scheme for calculating partial atomic charges in saturated systems. The parametrization of the Hückel coulomb and resonance integrals is given based on (a) the CNDO pi densities and (b) the observed molecular dipole moments. The results for these two schemes are compared both with other theoretical methods and also with the observed dipole moments of a variety of conjugated and aromatic compounds. The dipole moment (DM) parametrization gives in general more consistent results and is to be preferred. The compounds studied include substituted olefins and aromatics (with substituents R, F, Cl, Br, I, OR), aldehydes, ketones, acids, esters, amides, and the heterocyclic ring systems of furan, pyrrole, pyridine, and indole.