This paper uses the theory of atoms in molecules to investigate the origin of molecular dipole moments. The dipole moment is given by a sum over the net charge and first moment of every atom in a molecule. The first term leads to a charge transfer contributionJlc' the second to an atomic polarization contribution Jla' It is shown that both terms are, in general, of equal importance in determining both the static molecular dipole moment and the moment induced by a nuclear displacement. Models which imploy only point charges and corresponding bond moments which follow rigidly the nuclear framework, i.e., models which approximate Jlc and ignoreJla' are shown to lead to results that are incompatible with the changes that are found to occur in a molecular charge distribution during a nuclear vibration. The dipole moment is shown to be another group property that is transferable between molecules in the normal hydrocarbons, This property, along with the net charge, the energy, the correlation energy (expressed as a functional of the charge density), and the atomic volumes are transferable for methyl and methylene groups because of a corresponding transferability of the distribution of charge over the basins of the atoms in these groups up to their surfaces of zero flux in the gradient vector of the charge density. Transferability of group properties is not a result of the transferability of individual orbitals or geminals, which necessarily have tails lying outside of the group in question. Atoms have no tails and their energy, as is known from experiment, can in certain instances be transferred between systems with changes of less than 1 kcal/mol. The properties of atoms in molecules can be determined experimentally and quantum mechanics predicts these properties, just as it predicts the properties of a total system.
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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