Core-ionization energies and chemical properties

Abstract

Core-ionization energies can be related to the charge distribution in molecules and to the ability of a molecule to accept charge at a particular location. Typically, charge distributions are discussed in terms of point charges centered on the nuclei of the atoms in a molecule. Once appropriate corrections for relaxation energies have been made, the core-ionization energies can be related to these point charges via potential models. However, the concept of an atomic charge in a molecule is not well defined. Bader's method of integrating over basins of the charge distribution provides a method for defining such charges from the charge density, which is an observable quantity. Analysis of core-ionization energies in terms of Bader charges shows that the usual pointcharge model has serious drawbacks. Among these are that the charges on the atoms are not centered on the nuclei nor are they spherically distributed about the center of charge. Furthermore, the valence radius of an atom, which is taken to be constant in the point-charge model, depends significantly on both the atomic charge and the nature of the neighboring atoms. The point-charge model must, therefore, be replaced by one that includes the variability of the valence radius as well as the multipole nature of the charge distribution on the surrounding atoms. We find, however, that such a model even Including multipoles up to the octupole moment still does not give correct potentials and, hence, cannot provide an accurate picture of the effect of the charge distribution on the core-ionization energy. Core-ionization energies can be combined with gas-phase acidities, Auger kinetic energies, or results from ab initio calculations to show the influence of initial-state charge distribution and final-state relaxation on the ability of a molecule to accept charge at a specific location. Recent investigations of this nature have been concerned with the acidities of organic acids, such as carboxylic acids, phenol, and substituted phenols, with substituent effects in thiophene, and with factors that influence the rates of electrophilic addition. These have shown that traditional views about the effect of anionic resonance on acidities is incorrect. In addition, it is found that polarizable alkyl groups attached to electronegative centers affect the Initial-state potential as well as the final-state relaxation. For example, the higher acidity of formic acid relative to acetic acid arises because of this effect; the methyl group in acetic acid is more electron donating to the carbonyl carbon than is the hydrogen in formic acid. Investigations of substituted thiophenes show that there is significant resonance delocalization of electrons from the substituents to the ring in the neutral molecule and that the directing effect of the substituents is due to the initial-state charge distribution rather than delocalization of charge in the final state (or transition state). Relationships between experimental core-ionization energies and proton affinities (both experimental and theoretical) for fluoro-substituted ethenes illustrate the dominance of the initial-state potential over final-state relaxation in electrophilic addition reactions.