For quantitative description of a molecular charge distribution it is convenient to dissect the molecule into well-defined atomic fragments. A general and natural choice is to share the charge density at each point among the several atoms in proportion to their free-atom densities at the corresponding distances from the nuclei. This prescription yields well-localized bonded-atom distributions each of which closely resembles the molecular density in its vicinity. Integration of the atomic deformation densities — bonded minus free atoms — defines net atomic charges and multipole moments which concisely summarize the molecular charge reorganization. They permit calculation of the external electrostatic potential and of the interaction energy between molecules or between parts of the same molecule. Sample results for several molecules indicate a high transferability of net atomic charges and moments.

Bonded-atom fragments for describing molecular charge densities

A relationship between the charge capacity and the hardness of neutral atoms and groups

We introduce a polarity index Π that is proposed as a measure of the charge separation in a molecule. Π can be viewed as the average deviation of the electrostatic potential on the surface of the molecule. It permits a quantitative assessment of total local polarity, even in a molecule that has zero dipole moment. In support of our formulation of Π, we show that it is related to an empirical polarity-polarizability parameter and also to the dielectric constant.

Quantitative determination of the total local polarity (charge separation) in molecules

A general discussion of electronegativity and electronegativity equalization is presented as background for the concept of charge capacity; the latter is the ability of an atom to absorb or yield electronic charge. We have discussed charge capacity in relation to a variety of atomic and molecular properties, e.g. atomic polarizabilities and outer shell properties, electron affinities, dipole moments, electronic density distributions, dissociation energies, electrostatic potentials, spectroscopic measurements, acidities, etc. Significant increases in charge capacity and decreases in electronegativity in going from the first to subsequent rows of the periodic table are important in explaining (especially for columns IV–VII) the discontinuities in many physical and chemical properties that occur between these rows. It is demonstrated that both intrinsic electronegativities and charge capacities must be considered in order to understand the redistribution that occurs when atoms interact to form molecules. We discuss many seemingly anomalous observations of chemical behavior that are explained quite readily when both of these factors are taken into account.

Electronegativity and the concept of charge capacity

The electron projection function P(x, z) = ∫ ρ(x, y, z) dy is used to evaluate charge transfer and covalency in two series of molecules, LiX and CH3X (X = Li, BeH, BH2, CH3, NH2, OH, and F), with wavefunctions derived from STO-3G, 4-31G, and, in some cases, 6-31* ab initio calculations. The precision of the method and comparison with Mulliken populations analysis are described. Particular attention is given to CH3Li which by our criteria is wholly ionic.

Patricia H. Reggio

Papers

Properties of atoms in molecules. IV. Atomic charges in some linear polyatomic molecules