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Comparison of the successive ionization potentials in planar, nonaromatic hydrides with those in the corresponding perfluoro compounds demonstrates that MO's are stabilized 2.5-4 eV by the substitution, whereas the stabilization can be an order of magnitude smaller for MO's. This preferential stabilization of 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, and MO's are approximately equally stabilized by the fluorine atoms, showing that the trifluoromethyl group effectively destroys thedistinction in these molecules. Gaussian orbital calculations of double-^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 MO's are appreciably delocalized over the fluorine atoms, and are strongly stabilized by the high effective nuclear charge of that atom. In the r MO's, the delocalization onto the fluorine atoms is much less, and its stabilizing effect is counteracted by a strong antibond between the fluorine atom and the atom to which it is bonded.

The high-resolution He i and He ii photoelectron spectra of all fluoromethanes in the series CH4 to CF4 and their deuterated analogs have been recorded and are compared with the Koopmans' theorem results of near-Hartree–Fock calculations performed in a Gaussian basis. The agreement is very good in general and offers an unambiguous assignment of almost all of the bands observed. In particular, repeated correlations are demonstrated between the compositions of the orbitals from which the electrons are ejected and the characters of the resulting photoelectron bands. Identifiable trends throughout the series are stressed and an anomalous feature in the CF4 spectrum is noted. Jahn–Teller effects in CH4 and CH3F are clearly evident, but as expected, they are not observed in CHF3 and CF4. Comparison of the photoelectron spectra excited with He i and He ii radiation shows wide variations in the relative intensities of various bands in certain of the more symmetric molecules, suggesting that relative intensities can be a poor measure of relative orbital degeneracies. Mass-spectrometric appearance-potential data are briefly discussed in the light of the photoelectron results. The carbon and fluorine 1s binding energies as measured with 1254-eV x rays are shown to be electronically adiabatic. The accurate determination of the lower ionization potentials of these molecules leads readily to the assignment of several of their electronic transitions as lower members of ns and np Rydberg series.

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