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.

Principal component analysis is used to compare polar tensors of CH4, SiH4, GeH4, and SnH4 and their completely deuterated analogues determined from infrared fundamental gas-phase intensities measured in different laboratories. This analysis also includes theoretical polar tensor values obtained from effective core potential universal basis set calculations as well as from MP2/6-311++G(3d,3p) functions for CH4, SiH4, and SiF4. Theoretical values are also used to resolve sign ambiguities in the dipole moment derivatives of SiF4. Preferred polar tensor values are proposed for all these molecules. Mean dipole moment derivatives for SiH4 and SiF4 are related to the 2p core ionization energies using the simple potential model proposed by Siegbahn and collaborators. These results are confirmed by MP2/6-311++G(3d,3p) calculations for these molecules and for SiH3F, SiH2F2, and SiHF3. This study is extended to the fluorogermanes using experimental 3p and 3d core electron ionization energies and mean dipole moment derivatives calculated from MP2/A-VDZ/6-311++G(3d,3p) wave functions. The simple potential model interpretation of mean dipole moment derivatives as atomic charges implies that the silicon charge, ±0.904e, is slightly higher than the germanium charge of ±0.862e.

The simple potential model has been shown to be useful in relating core electron binding energies measured in the X-ray region with mean dipole moment derivatives obtained from experimental infrared vibrational intensities. The importance of including relaxation corrections to the experimental 1s ionization energies of sp, sp 2 , and sp 3 hybridized carbon atoms are investigated here. Although relaxation energies obtained from 6-31G(d,p) and 6-311++G(3df,3p) basis sets using ∆SCF calculations show differences of about 1 eV for most molecules studied, relative differences are of the order of 0.1 eV. Exceptions are the CO, CO 2 , COS, and CS 2 molecules where discrepancies are larger. Relaxation energy corrections improve simple potential model fits with mean dipole moment derivatives for all carbon atom models but is most pronounced for the sp hybridized atoms. The simple potential model corrected for relaxation energies is investigated as a criterion for testing the quality of Mulliken, CHELPG, Bader and GAPT carbon atomic charges calculated from MP2/ 6-311++G(3d,3p) wave functions. The GAPT charges are in excellent agreement with the experimental mean dipole moment derivatives (within 0.067e) and provide superior statistical fits to the simple potential model when compared with those obtained for the other charges.

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