No abstract

An interpretation of the electronic spectrum of Prussian blue based on ligand field theory is presented which is consistent with the dye having the constitutional formula KFenl[Fen(CN)6]. The blue color of the dye arises from a charge transfer transition between the Fen(CN)e4*~a nd Fem ions; the measured intensity of the transition indicates that the optical electrons are in very large part (99%) localized on the Fen(CN)64-ions in the ground state. The interpretation of the shift of the high frequency charge transfer bands of Fen(CN)e4_ on substituting the Fe11 with Ru11 or Os11 then can be used to predict the shifts of the low frequency charge transfer bands of Prussian blue on substituting Fe'ffCNh4-with Run(CN)e4_ or Osn(CN)e4-.

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.

No abstract

Electron transmission, inner-shell electron energy loss and magnetic circular dichroism spectra have been analyzed in an effort to trace the positions of the * antibonding valence MOs in benzene and its fluorinated derivatives. The correlation of negative-ion resonances in these systems shows clearly that a a* valence level descends with increasing fluorination so as to become the lowest virtual MO in hexafluorobenzene. This is understandable in terms of the perfluoro effect acting upon virtual MOs in a way parallel to that known to occur for occupied MOs. In addition to the low-lying a* negative-ion shape resonances, several negative-ion Feshbach resonances are identified as involving 3s and 3p Rydberg orbitals. The search for low-lying * levels in heavily fluorinated benzenes is extended to their C Is and F Is inner-shell spectra. The carbon K-shell spectra of benzene and its fluorinated derivatives below the respective C Is ionization potentials are dominated by excitations to 1 * and 2 * valence levels. In the C Is spectra of pentafluoro-and hexafluorobenzene, additional low-lying bands are observed and assigned to C ls(C-F) -* a*(C-F) transitions. Spectral stripping indicates the location of the corresponding (C ls(C-F)-1, *) states in the spectra of the other fluorobenzenes. A systematic shift of these * levels to lower energy with increasing fluorination is observed which is consistent with the perfluoro effect. Resonances terminating at ff*(C-C) are found to dominate the C Is near continuum, with dramatic enhancement of these transitions in the more highly fluorinated species. Investigation of hexafluoroand 1,2,4,5-tetrafluorobenzene by vacuum-ultraviolet magnetic circular dichroism in the vapor phase confirms the presence of bands which are not tt -> tt*. Once again, low-lying * MOs are invoked as terminating orbitals.

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The first four bands in the gas-phase spectra of amides, carboxylic acids, and acyl fluorides are thought to be n → π*, n → 3s Rydberg, π → π*, and n → 3p Rydberg excitations. That the second and fourth bands are Rydberg, whereas the first and third are valence shell is demonstrated in a comparison of gas-phase and condensed-phase absorption and circular dichroism spectra. All-electron, SCF Gaussian orbital calculations are also presented which qualitatively explain the trends in the spectra of HCOX molecules, and predict several quantities of interest, such as upper-state dipole moments and magnetic transition moments, which have not been measured as yet.