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Item Open Access Absolute Oscillator Strengths From the K-Shell Spectra of Fluoroethenes and 1,3-perfluorobutadiene(American Physical Society, 1987) McLaren, R.; Clark, S.A.C.; Ishii, I.; Hitchcock, A.P.Absolute oscillator strengths in the region of carbon and fluorine K-shell excitation have been derived for CH2CH2, CH2CHF, cis-CHFCHF, CH2CF2, CHFCF2, CF2CF2, 1,3-C4H6, and 1,3-C4F6 from electron-energy-loss spectra recorded under dipole-dominated conditions. The methods used to derive absolute oscillator strengths from relative energy-loss intensities are discussed in detail. The accuracy of the procedures is tested through comparisons with literature results for N2, CO, and CO2. The total C 1s→π* and C 1s→σ*(C—F) intensities increase systematically as the degree of fluorination increases. The spectra are discussed in terms of bond-length correlation and potential barrier concepts.Item Open Access Fluorination Effects on the Inner-Shell Spectra of Unsaturated Molecules(Elsevier, 1988) Robin, M.B.; Ishii, I.; McLaren, R.; Hitchcock, A.P.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.Item Open Access The * Molecular Orbitals of Perfluoroalkanes as Studied by Inner-Shell Electron Energy Loss and Electron Transmission Spectroscopies(National Research Council Canada, 1988) Ishii, I.; McLaren, R.; Hitchcock, A.P.; Jordan, K.D.; Choi, H.; Robin, M.B.Absolute oscillator strength spectra in the C 1s (280–340 eV) and F 1s (680–740 eV) regions of the perfluoro-n-alkanes from C2 to C6 and perfluorocycloalkanes from C3 to C6 have been determined from inner-shell electron energy loss spectra recorded under electric-dipole scattering conditions. The spectral features are interpreted in terms of spatially localized transitions terminating at orbitals of predominantly σ*(C—F) and σ*(C—C) character. When compared to the spectra of the perfluoro-n-alkanes, both the C 1s and F 1s spectra of the perfluorocycloalkanes exhibit additional low-lying bands which are assigned to transitions terminating at σ*(C—C) orbitals which are shifted to low energy by the combination of the strain of cyclization and the inductive effect of the fluorination. The electron transmission spectra of selected perfluorocycloalkanes (which provide information on their anion states) show as well that the electron affinities of the cyclic systems are substantially lower than those of the corresponding perfluoro-n-alkanes, again as a result of a low-lying σ* orbital in the cyclic species. Quantum chemical calculations of the alkane and perfluoroalkane ground-state orbital structures support the experimental results. The localized character of the inner-shell excitations, indicated by the constancy of both term values and oscillator strengths with increasing chain length, contrasts with the more delocalized character of the states accessed in ultraviolet excitation or negative ion formation.