Perfluoro effect in photoelectron spectroscopy. I. Nonaromatic molecules

Abstract

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 X̃ 2B1, B22, A12, and A22 states of oxygen difluoride cation (F2O+): High-level ab initio calculations and simulation of the ultraviolet photoelectron spectrum of F2O

“…Regarding the alternative assignment of the position of the 2 A 2 state mentioned by a͒ Brundle and co-workers, 2 some calculated VIEs ͑e.g., those obtained by the ROCIϩQ, LDA-rTS and pGW2 methods; see Table I͒ clearly support it, but for the others, there is no clear preference between the original 1,2 and the alternative assignments. 2 The computed value obtained from the highest level of calculation prior to the present study is that obtained by Valenta and co-workers 3 from a MRDCI study. Their VIE value of 17.41 eV for ionization to the 2 A 2 state can be considered to favor the original assignment of associating the 2 A 2 ionic state to the broad band at 18.50 eV.…”

“…Ab initio calculations were performed, and both the Koopmans' Theorem ͑KT͒ and the ⌬ self-consistent field ͑SCF͒ method were utilized to assign the observed spectrum. The assignments of the first four PE bands from these two studies are essentially identical, giving the order of the lowest four states of F 2 Table I͒. However, Brundle and co-workers 2 also suggested an alternative assignment.…”

“…Some of the computed VIEs reported for the lowest four cationic states [1][2][3][4][5][6][7][8][9] are summarized in Table I ͑see footnote a͒. From Table I, it is clear that the assignment of the 2 B 1 state of F 2 O ϩ to the first observed PE band is reasonably certain.…”

“…In view of this consideration, the 2 A 2 ionic state might be more appropriately assigned to the relatively sharp third PE band at 16.44 eV, rather than the broad and relatively weak band at a significantly higher VIE of 18.50 eV. Thus, the 2 ϩ are unclear, as three states are now assigned to two observed bands with no resolved vibrational structure.…”

Iodopentafluorobenzene: Electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption and photoelectron spectroscopy

“…This confirms the assignment to the iodine atom lone pair molecular orbitals. They are shifted to higher energies compared to iodobenzene [17] due to the presence of fluorine atoms [18].…”