Charge calculations in molecular mechanics 7: application to polar pi systems incorporating nitro, cyano, amino, C=S and thio substituents.

A previously described method for calculating partial atomic charges in molecules has been applied to a variety of different functional groups with pi electrons. The pi charges have been determined by varying the relevant Hükel coulombic and resonance integrals so as to reproduce the experimental dipole moments of known compounds. These include nitro, cyano, mercapto and thiocarbonyl compounds, anilines and a series of heterocyclic ring compounds involving nitrogen, oxygen and sulphur atoms. The calculated dipole moments are in good agreement with experimental values and are a considerable improvement on other calculated values, while requiring only a fraction of the computational time necessary for more rigorous methods. The resulting pi densities compare favourably with other theoretical (CNDO, MNDO and STO-3G) methods.