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| - The Substituent Effect of the Methyl Group. Carbon 1s IonizationEnergies, Proton Affinities, and Reactivities of the Methylbenzenes
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| - 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 expectedin the spectra, it has been possible for the first time to assign 1s ionization energies to each of theinequivalent carbon atoms in these molecules. There exist linear correlations between the ionization energiesand the energy changes for other chemical processes, such as enthalpies of protonation and activationenergies for hydrogen exchange and protodesilylation. There are deviations from these correlations forsites in which hyperconjugation plays a role in the process. These can be understood by recognizing thatthe core-ionization energies reflect primarily the Hammett parameter σ whereas the other energies reflectσ+. The ionization and reaction energies can be summarized compactly with a linear model in which thetotal effect of the substituents is equal to the sum of the effects of the individual substituents. A slightlybetter description is obtained with a quadratic model, which allows for interaction between the substituents.
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