Electronic structure of the fluorobenzenes, ethylene, and tetrafluoroethylene

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Abstract
The electronic structures of C6H6‐nFn, 1⩽n⩽6, C2H4, and C2F4 are evaluated by extending the new ’’spectroscopic’’ self‐consistent‐field CNDO/S2 molecular orbital model to include a parameterization of fluorine. As in our earlier studies of conjugated hydrocarbons, the CNDO/S2 model provides an adequate description of photoelectron spectra associated with states of ionization potential I≲13 eV. The intensities of vibrational progressions evident in the photoelectron spectra are analyzed to extract the linear coupling constants of electrons in the various molecular orbitals to the totally symmetric (a1g) molecular vibrations and, when allowed by symmetry, to e2g molecular vibrations. These coupling constants differ substantially from those obtained for benzene and deuterobenzene. Their values reflect the consequences of the heavy fluorine mass and the incompressibility of the charge density on the fluorine substituent. They lead, moreover, to explicit predictions of increased insulating behavior for fluorobenzenes relative to benzene in the solid state. Analyses of the ultraviolet absorption spectra of the fluorobenzenes are less satisfactory than in the previously considered cases of conjugated hydrocarbons. The fluorine substituents produce ionic virtual orbitals whose greater diffuseness relative to the occupied orbitals must be incorporated into the model in order to achieve a quantitative (ΔE⩽0.1 eV) description of all of the fluorobenzene ultraviolet absorption spectra. The singlet absorption spectrum of C2H4, however, is described quantitatively; a surprising result considering the simplicity of the CNDO/S2 model.

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