Simple potential models for carbon 1s ionization energies using infrared mean dipole moment derivatives

Simple potential model relations for experimental carbon Is ionization ener gies (E-C,E-1s) and carbon mean dipole moment derivatives ((p) over bar(C)) obtained from experimentally measured infrared fundamental band intensitie s are investigated for a diverse group of 29 molecules. Positive and negati ve correlations of the E-C,E-1s values and neighboring atom electrostatic p otential contributions, V, with the (p) over bar(C) values result in large variances for the E-C,E-1s-V values and excellent potential model fits. MP2 /6-311+ +G(3d,3p) level Koopmans' energies are shown to provide the most pr ecise potential model fits with correlation coefficients of 0.9996, 0.9962 and 0.9960 for sp(3), sp(2) and sp hybridized carbon atoms, respectively. P otential models using experimental ionization energies adjusted by HF/6-31G (d,p) level relaxation energies are almost as precise. The slopes of the po tential lines obtained using Koopmans' energies or experimental ionization energies adjusted by relaxation energies increase with increasing values of the inverse covalent sp(3), sp(2) and sp radii. Relative electrostatic pot entials at carbon nuclei calculated directly from electronic densities of M P2/6-311+ +G(3d,3p) molecular orbital wave functions are shown to be in goo d agreement with those estimated by mean dipole moment derivatives calculat ed from the same wave functions. (C) 2000 Elsevier Science B.V. All rights reserved.