The theory of atoms in molecules is used to analyze ab initio electron distributions for series of simple ethyl and formyl derivatives. The dipole moments of these molecules are decomposed into their unique charge-transfer and atomic polarization contributions, as defined by the theory. It is shown how atomic charges are related by quantum mechanics to the observable dipole moments of molecules. Substituent effects on atomic charges and hence on dipole moments are described. An analysis of the atomic populations shows that the calculated behavior of substituents on the electron density of the formyl and ethyl groups is in accord with the predictions of qualitative orbital models, employing a separation of and it substituent effects. This paper thus gives quantitative expression to those models, by using a theory with a firm quantum mechanical basis. It is suggested that substituent constants can be extracted from the theory of atoms in molecules in an empirical manner. Substituents affect the atomic charges in ethyl and formyl derivatives in contrasting ways, and a perturbation model of changes to the electron density is used to explain this different behavior.
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