The Substituent Effect of the Methyl Group. Carbon 1s Ionization Energies, Proton Affinities, and Reactivities of the Methylbenzenes

Abstract

High-resolution carbon 1s photoelectron spectra have been measured for methyl-substituted benzenes. By using these data together with molecular structure calculations to predict the vibrational profiles expected in the spectra, it has been possible for the first time to assign 1s ionization energies to each of the inequivalent carbon atoms in these molecules. There exist linear correlations between the ionization energies and the energy changes for other chemical processes, such as enthalpies of protonation and activation energies for hydrogen exchange and protodesilylation. There are deviations from these correlations for sites in which hyperconjugation plays a role in the process. These can be understood by recognizing that the core-ionization energies reflect primarily the Hammett parameter sigma whereas the other energies reflect sigma+. The ionization and reaction energies can be summarized compactly with a linear model in which the total effect of the substituents is equal to the sum of the effects of the individual substituents. A slightly better description is obtained with a quadratic model, which allows for interaction between the substituents.

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.…”

“…Two papers have been published and one submitted concerning substituent effects in substituted benzenes. The first of these has dealt with fluorine as a substituent, 1 the second with the methyl group as a substituent, 2 and the third on the combined effects of both fluorine and the methyl group. 3 In these papers we have shown that the effects of multiple substituents on not only carbon 1s ionization energies, but also reactivities and enthalpies of protonation, can be accounted for primarily by a linear additivity model.…”

“…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.…”

Accurate calculation of C1s core electron binding energies of some carbon hydrates and substituted benzenes

“…Atom ical approach that can reproduce the adiabatic and vertical C1s ionization energies of the molecules, observed [16][17][18][19][20][21] with the new generation synchrotrons, as accurately as possible.…”

“…Recently, accurately observed values of adiabatic and vertical C1s ionization energies of alkanes [16], alkenes [17], alkynes [18], fluorobenzenes [19], methylbenzenes [20], and fluoromethylbenzenes [21] have been published. All the experimental values were measured using new generation synchrotorons.…”