On the charge distribution in ethanes and disilanes and correlations with equilibrium bond lengths; an ab initio study

The equilibrium electronic wave-functions for a series of fluoro- and chlor o-ethanes and disilanes of general formula M2H6-nXn, (M = C, Si; X = F, Cl) : were analysed by the most commonly used methods for electron distribution , using the Mulliken and Lowdin populations, natural atomic orbital (NAO) p opulations and atoms in molecules (AIM) electron densities. Although the nu merical values for local atomic charges vary greatly, all the methods corre late, but in markedly differing ways. The Mulliken charges seem the most se lective in relation to systematic change of substituents in the current typ e of molecular structure. A number of examples occur where the AIM charges at C, Si centres are effectively identical in different molecules, where so me differences might have been anticipated. These are often distinguished b y Mulliken populations. The fluoroethanes exemplify this, since a plot of t he AIM charges (for example on either the F or H centres) against the Mulli ken charges for all members of the series, shows three nearly parallel line s, corresponding to those centres with 0, 1 or 2 fluorine atoms on the cent re under study. The bond critical points at which the AIM charges are deter mined seem to be counter to intuition in some cases. This is a density rath er than atomic orbital size issue however. The Mulliken and NAO charges see m more reasonable than those from the AIM method. There is an unexpected co rrelation of the local bond dipoles from the Mulliken analyses, with the ca lculated equilibrium bond lengths. These correlations lead to bond length v alues for the non-polarised bonds MX, which agree with data based on covale nt radii for some bonds. (C) 2000 Elsevier Science B.V. All rights reserved .