Additivity of Substituent Effects. Core-Ionization Energies and Substituent Effects in Fluoromethylbenzenes

Abstract

Carbon 1s ionization energies have been measured for all of the carbon atoms in eight fluoromethylbenzenes. Enthalpies of protonation have been calculated for protonation at all of the ring carbons in the same molecules. These data together with previously reported data on fluorobenzenes and methylbenzenes provide the basis for studying the additivity of substituent effects and the correlation between enthalpies of protonation with core-ionization energies. Although a linear additivity model accounts reasonably well for both the ionization energies and the enthalpies of protonation, a better description, especially for the enthalpies, is obtained by inclusion of nonlinear terms that account for interactions between two substituents on the same molecule. There are families of nearly parallel correlation lines between enthalpies of protonation and core-ionization energies. The existence of several families can be primarily understood in terms of the linear additivity picture and more completely understood when the nonlinear terms are taken into account. The role of the methyl group as a polarizible pi-electron donor is contrasted with the role of fluorine, which is a substituent of low polarizibility that acts to withdraw electrons from the adjacent carbon and to donate electrons through resonance to the ring. The role of the hydrogen atoms as pi-electron acceptors in the protonated species is illustrated.

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.…”

“…The authors who published the observed Table 5 Calculated and observed adiabatic and vertical C1s core electron binding energies, in eV, of some methyl-fluoro-substituted benzenes without (C rel = 0 eV) and with (C rel = 0.05 eV) relativistic correction. [21]. b Average absolute deviation in eV.…”

“…Systematic errors of the absolute IP are cancelled out when calculating relative energies. Approach ADI and Approach VERT, which we are using, can calculate absolute (as well as relative) CEBE, while the procedure adopted by the previous authors [19][20][21] can calculate only the relative CEBE. The accuracy of the absolute CEBE calculated is as good as that for the relative CEBE.…”

“…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.…”

“…CEBE of the substituted benzenes[19][20][21] also published calculated relative CEBE values of C1s with respect to C1s of benzene. The procedure used for their theoretical calculations was the DFT method using B3LYP functional with a triple-f basis set plus polarization functions.…”