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

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 σ whereas the other energies reflect σ+. 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.