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| - Substituent Effects on the Acidity of Weak Acids. 2. CalculatedGas-Phase Acidities of Substituted Benzoic Acids
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| - To investigate the origin of substituent effects on the acidity of benzoic acids, the structures of aseries of substituted benzoic acids and benzoates have been calculated at the B3LYP/6-311+G*and MP2/6-311+G* theoretical levels. The vibrational frequencies were calculated using B3LYP/6-311+G* and allowed corrections for the change in zero-point energies on ionization, and the changein energy on going from 0 K (corresponding to the calculations) to 298 K. A more satisfactoryagreement with the experimental values was obtained by energy calculations at the MP2/6-311++G** level using the above structures. The resulting ΔHacid values agree very well with theexperimental gas-phase acidities. The energies of compounds with π-electron-accepting or -releasingsubstituents, rotated to give the transition state geometries, provided rotational barriers thatcould be compared with those found for the corresponding substituted benzenes. Isodesmic reactions allowed the separate examination of the substituent effects on the energies of the acids andon the anions. Electron-withdrawing groups stabilize the benzoate anions more than they destabilize the benzoic acids. Electron-donating groups stabilize the acids and destabilize the anions byapproximately equal amounts. The gas-phase acidities of meta- and para-substituted benzoicacids are linearly related. This is also found for the acidities of substituted phenylacetic acids andbenzoic acids. Since direct π-electron interactions are not possible with the phenylacetic acids,this indicates that the acidities are mainly controlled by a field effect interaction between thecharge distribution in the substituted benzene ring and the negative charge of the carboxylategroup. The Hammett σM and σP values are also linearly related for many small substituents fromNO2 through the halogens and to OH and NH2. Most of the other substituents fall on a line witha different slope
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