Perfluoro Effect in Photoelectron Spectroscopy. I. Nonaromatic Molecules

Comparison of the successive ionization potentials in planar, nonaromatic hydrides with those in the corresponding perfluoro compounds demonstrates that σ MO's are stabilized 2.5–4 eV by the substitution, whereas the stabilization can be an order of magnitude smaller for π MO's. This preferential stabilization of σ MO's is termed “the perfluoro effect.” The generality of the perfluoro effect was demonstrated experimentally and theoretically using the ethylene-tetrafluoroethylene, water-oxygen difluoride, formaldehyde-carbonyl fluoride, and diimide-difluorodiazine pairs. He(I) and He(II) photoelectron spectra of all of these molecules except diimide are presented, together with those on the acetone-hexafluoroacetone, azomethane-hexafluoroazomethane, and butadiene-1,1,4,4-tetrafluorobutadiene pairs. In the pairs containing the methyl and trifluoromethyl groups, σ and π MO's are approximately equally stabilized by the fluorine atoms, showing that the trifluoromethyl group effectively destroys the π–σ distinction in these molecules. Gaussian orbital calculations of double-ζ quality were performed for the smaller molecular pairs; the Koopmans' theorem values are in good agreement with experiment. Analysis of the wave functions shows that in the perfluoro compounds, the σ MO's are appreciably delocalized over the fluorine atoms, and are strongly stabilized by the high effective nuclear charge of that atom. In the π MO's, the delocalization onto the fluorine atoms is much less, and its stabilizing effect is counteracted by a strong π antibond between the fluorine atom and the atom to which it is σ bonded.