Fluorination Effects on the Inner-Shell Spectra of Unsaturated Molecules

The applicability of the perfluoro effect to the X-ray spectra (300–800 eV) of unsaturated organic molecules is explored. The C1s and F1s (and Ols where appropriate) oscillator strength spectra of five fluoroethylenes, octafluorocyclopentene, formyi fluoride, carbonyl fluoride, hexa-fluorobutadiene, trifluoroacetic acid, hexafluorobutyne-2, hexafluoroacetone, and octafluorona-phthalene were derived from electron impact energy loss spectra recorded under electric-dipole scattering conditions. These spectra are analyzed and compared with those of their perhydro analogs, several of which (naphthalene, acetic acid, butyne-2) are reported for the first time. In unsaturated systems in which all the atoms lie in the molecular plane, such as ethylene, formaldehyde, benzene, etc., perfluorination results in approximately 10 eV shifts of the inner-shell energy loss spectra to higher energies, yet the term values for the C1s→1π* excitations are shifted by only 1 eV and often less. In direct contrast, the term values for the equivalent C1s→1π* excitations in unsaturated systems having atoms out of the molecular plane, such as butene-2 and acetone, are shifted upward by up to 3 eV upon perfluorination. These different spectral behaviors of planar and nonplanar systems on fluorination quantitatively parallel those which were observed earlier for valence-level ionization potentials (10–20 eV) and attributed to the perfluoro effect. It is observed for the first time that the C1s→1π* excitation energies in planar hydrocarbons are only very weakly dependent on the spatial extent of the π-electron system. An explanation involving a localized C1s hole is proposed to rationalize this behavior.

The perfluoro effect also predicts that excitations to σ* MO's will become relatively low-lying in highly fluorinated planar systems. Such low-lying inner-shell excitations induced by fluorination are observed in the fluoroethylene series and in the fluorocarbonyls. When the negative-ion spectra of the fluoroethylenes are assigned in a self-consistent manner, a σ* MO is found to drop into the vicinity of 1π* upon fluorination. A similar intrusion of the lowest σ MO among the π* MO's is also observed upon fluorinating benzene, while evidence for this in the case of naphthalene is less clear, on account of the complex pattern of multiple C1s→nπ* transitions in this molecule.

Inner-shell oscillator strength distributions are reported for all the spectra considered herein. In general, perfluorination increases the oscillator strengths of C1s→1π* transitions by up to a factor of two. Variation of the C1s→1π* and O1s→17π* oscillator strengths in the series H2CO, HFCO, F2CO shows clearly how the 17π* MO becomes more polarized toward C as fluorination proceeds. In some cases, C1s→σ* (C-F) oscillator strengths exceed those for C1s→1π* transitions.