Core Electron Energies, Infrared Intensities, and Atomic Charges

Abstract

Carbon 1s electron binding energies determined by X-ray photoelectron spectroscopy and mean dipole moment derivatives obtained from experimental infrared intensities are shown to be related through the simple potential model proposed by Siegbahn and collaborators. The sp3 carbon atoms in 13 halomethanes, 2 ethanes, 3 methylacetylenes, cyclopropane, and ethylene oxide have 1s energies, which, after correction for electrostatic potentials from neighboring atoms, are linearly related to the carbon mean dipole moment derivatives, presenting a slope of 15.50 ± 0.29 eV/e. The sp2 carbons of ethylene, three haloethylenes, and three carbonyl compounds also exhibit a linear relationship having a significantly different slope of 17.37 ± 0.87 eV/e. The sp carbon atoms in acetylenes, cyanides, CO, CS2, CO2, and OCS show a third linear relationship, with a slope of 18.90 ± 0.75 eV/e. These slopes are proportional to the inverse atomic radii of sp3, sp2, and sp carbon atoms and according to the simple potential equation can be interpreted as estimates of Coulomb repulsion integrals involving these hybridized orbitals and the 1s core electron orbitals. Two basic assumptions of the potential model are investigated. The effect of relaxation energies on the 1s electron ionization processes is estimated as the difference between ΔSCF ionization energies and Koopmans' frozen orbital estimates obtained from 6-31G(d,p) wave functions. These results are compared with values obtained previously from the equivalent cores estimating procedure. Also the conceptual validity of identifying the carbon mean dipole moment derivatives as atomic charges is discussed within the framework of the charge−charge flux-overlap model.