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Application of the Equivalent Bond Orbital Model to the C 2s ‐Ionization Energies of Saturated Hydrocarbons
Author(s) -
Bieri Gerhard,
Dill James D.,
Heilbronner Edgar,
Schmelzer Andreas
Publication year - 1977
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19770600715
Subject(s) - chemistry , atomic orbital , ionization , molecular orbital , ionization energy , parametrization (atmospheric modeling) , computational chemistry , atomic physics , molecular physics , molecule , physics , quantum mechanics , electron , ion , organic chemistry , radiative transfer
It is shown that the ab initio STO‐3G treatment applied to simple saturated linear, branched and cyclic hydrocarbons, assuming standard geometries, yields orbital energies ϵ j STO‐3Gfor their canonical orbital φ j which correlate perfectly with the observed C 2s ionization energies I j m , if Koopmans ' approximation is accepted. Applying the Foster ‐ Boys localization procedure to these canonical orbitals φ j leads to localized orbitals λμ and their corresponding Hartree ‐ Fock matrix F λ = (F λ,μv ). An examination of the matrix elements F λ,μv , i.e. of the self‐energies A μ = F λ,μμ of the localized CC‐ and CH‐orbitals λ μ and of the cross terms F λ, μv (μ ≠ v) between them, leads to the conclusion that a satisfactory approximation should be obtained by setting A μ = A for all μ, F λ,μv = B if λ μ and λ v are vicinal and neglecting all other cross terms. The resulting model is nothing but the well‐known equivalent bond orbital model of Lennard ‐ Jones & Hall , which however can now be calibrated using the known C 2s ‐ionization energies of hydrocarbons. Due to the discrete structure and the wider range (∽ 8 eV) of the C 2s band systems in the photoelectron spectra of these molecules this leads to a more satisfactory parametrization than using the narrower and badly resolved C 2p band system. Comparison of calculated band positions using the calibrated model with observed C 2s ‐band ionization energies for a series of hydrocarbons reveals that the simple equivalent bond orbital model is better than one might have expected.