Core Ionization Energies, Mean Dipole Moment Derivatives, and Simple Potential Models for B, N, O, F, P, Cl, and Br Atoms in Molecules

Abstract

Simple potential models relating experimental 1s electron ionization energies for B, N (sp and sp3 hybrids), O, and F atoms; 1s and 2p ionization energies for P atoms; and 2s and 2p ionization energies for Cl atoms as a function of their atomic mean dipole moment derivatives determined from experimental gas phase infrared fundamental band intensities are reported. Potential models using theoretical Koopmans' energies and generalized atomic polar tensor (GAPT) charges are found to form even more precise models than those using experimental data. This is expected because the potential models depend only on the electronic structures of molecules before ionization takes place and do not take into account relaxation effects. If the experimental ionization energies are adjusted by their relaxation energies, models similar to those obtained using Koopmans' energies are determined. The models permit a simple understanding of substituent effects on core ionization energies in terms of atomic charges in molecules. Most of the potential model slopes investigated are shown to be approximately proportional to the inverse atomic radii of the atom being ionized. Core-valence electron repulsion values inferred from the potential models obtained from experimental data are somewhat smaller than those calculated using Slater orbitals of isolated atoms. The potential model intercepts for 1s and 2p electrons are shown to be proportional to the square of the nuclear charge, consistent with their interpretation as core electron ionization energies of neutral atoms. 1s He, Ne, and Ar and 2p Ar, Kr, and Xe core ionization energies obey the linear relationships obtained for the model intercepts. The results suggest that mean dipole moment derivatives obtained from infrared intensities can be interpreted as atomic charges.