Energies of the gas-phase deprotonation of nitro-substituted benzenesulfonic and benzoic acids: The role of the conformation isomerism of sulfonic acids

Abstract

The Gibbs energies of deprotonation Δr G 298 ○ of gaseous benzoic acid (BA), benzenesulfonic acid (BSA) and their six mono-, di-, and trinitro-substituted derivatives are calculated by means of B3LYP/cc-pVTZ and MP2/6-311++G**. The dependences of Δr G 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 ortho-substituted 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 Δr G 298 ○ values for substituted BA are ∼22 kcal/mol higher than the corresponding values for substituted BSA. A trend of diminishing Δr G 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.