Energies of the Gas-Phase Deprotonation of Nitro-Substituted Benzenesulfonic and Benzoic Acids: The Role of the Conformation Isomerism of Sulfonic Acids

摘要

The Gibbs energies of deprotonation ?_rG_(298)~° of gaseous benzoic acid (BA), benzenesulfonic acid (BSA) and their six mono-, di-, and trinitro-substituted derivatives are calculated by means of B3LYP/ccpVTZ and MP2/6-311++G**. The dependences of ?_rG_(298)~°on the number and the position of nitro groups in an aromatic ring are revealed, as is the possibility of intramolecular hydrogen bond (IHB) formation in orthosubstituted acids. It is found that the deprotonation of conformers of ortho-nitro-substituted BSA without IHBs requires less energy (by 4-5 kcal/mol) than for conformers with IHBs. It is shown that the ?_rG_(298)~° values for substituted BA are ~22 kcal/mol higher than the corresponding values for substituted BSA. A trend of diminishing ?_rG_(298)~° for nitro-substituted acids is observed when the number of nitro groups is increased, and di- and trinitro-substituted BSA may therefore be considered superstrong acids.