Fluorine as a π Donor. Carbon 1s Photoelectron Spectroscopy and Proton Affinities of Fluorobenzenes

Abstract

The carbon 1s ionization energies for all of the carbon atoms in 10 fluorine-substituted benzene molecules have been measured by high-resolution photoelectron spectroscopy. A total of 30 ionization energies can be accurately described by an additivity model with four parameters that describe the effect of a fluorine that is ipso, ortho, meta, or para to the site of ionization. A similar additivity relationship describes the enthalpies of protonation. The additivity parameters reflect the role of fluorine as an electron-withdrawing group and as a pi-electron donating group. The ionization energies and proton affinities correlate linearly, but there are four different correlations depending on whether there are 0, 1, 2, or 3 fluorines ortho or para to the site of ionization or protonation. That there are four correlation lines can be understood in terms of the ability of the hydrogens at the site of protonation to act as a pi-electron acceptor. A comparison of the ionization energies and proton affinities, together with the results of electronic structure calculations, gives insight into the effects of fluorine as an electron-withdrawing group and as a pi donor, both in the neutral molecule and in response to an added positive charge.

Inner-shell single and double ionization potentials of aminophenol isomers

“…these states exhibit positive chemical shifts, 39 in phenol only two states from this group have IPs larger than those of benzene and none of them has positive chemical shift in aniline which is in accord with the electron withdrawing power of the corresponding substituents. Note that the carbon atoms in the para and ortho positions relative to the site of ionization have lower IPs (the energy lowering is stronger in the para case) than the carbons in the meta positions due to resonance effects.…”

“…For comparison, the chemical shift associated with the carbon atom adjacent to fluorine in fluorobenzene is 2.44 eV. 39 These chemical shifts correlate with the electronegativity along the series NH 2 < OH < F.…”

“…4 and also given in Table II. Often the effect of multiple substituents, e.g., on the chemical shift, is described in terms of an additivity model 39 where the total effect of the substituents is represented as a sum of independent effects of the individual substituents. We did not inspect thoroughly the performance of this model in the case of aminophenol since this is beyond the scope of the present paper.…”

Inner-shell electron spectroscopy and chemical properties of atoms and small molecules

“…For 1,3-butadiene and 1,3-pentadiene we have shown that, in keeping with this expectation, there are such linear correlations between the carbon 1s ionization energies and the activation energies for addition of HCl at various positions in the molecule and between the carbon 1s ionization energies and the enthalpies of protonation. 4 The studies of the fluoro-and methyl-substituted benzenes 1,2,3 show similar linear correlations, but in some cases there are significant deviations of the data from a single correlation line. In particular, for the correlations between carbon 1s ionization energies and enthalpies of protonation there are four separate but nearly parallel lines for compounds that have 0, 1, 2, or 3 fluorine substituents that are either ortho or para to the site of ionization/protonation.…”

“…In the papers discussed above, [1][2][3] as well as in an additional paper on carbon 1s ionization energies in conjugated aliphatic systems, 4 we have considered the relationships between carbon 1s ionization energies and other chemical properties, such as enthalpies of protonation and activation energies for electrophilic reactions. All of these processes involve the addition of a positive charge at a localized point in a molecule and it is expected that the energy changes that accompany one of these will be linearly related to the energy changes for another.…”