| Summary: | Structural factors have been identified that determine the gas-phase acidity of <i>ortho</i>-substituted benzenesulfonic acid, 2-XC<sub>6</sub>H<sub>4</sub>–SO<sub>3</sub>H, (X = –SO<sub>3</sub>H, –COOH, –NO<sub>2</sub>, –SO<sub>2</sub>F, –C≡N, –NH<sub>2</sub>, –CH<sub>3</sub>, –OCH<sub>3</sub>, –N(CH<sub>3</sub>)<sub>2</sub>, –OH). The DFT/B3LYP/cc-pVTZ method was used to perform conformational analysis and study the structural features of the molecular and deprotonated forms of these compounds. It has been shown that many of the conformers may contain anintramolecular hydrogen bond (IHB) between the sulfonic group and the substituent, and the sulfonic group can be an IHB donor or an acceptor. The Gibbs energies of gas-phase deprotonation Δ<sub>r</sub>G<sup>0</sup><sub>298</sub> (kJ mol<sup>–1</sup>) were calculated for all compounds. It has been set that in <i>ortho</i>-substituted benzenesulfonic acids, the formation of various types of IHB is possible, having a significant effect on the Δ<sub>r</sub>G<sup>0</sup><sub>298</sub> values of gas-phase deprotonation. If the –SO<sub>3</sub>H group is the IHB donor, then an ion without an IHB is formed upon deprotonation, and the deprotonation energy increases. If this group is an IHB acceptor, then a significant decrease in Δ<sub>r</sub>G<sup>0</sup><sub>298</sub> of gas-phase deprotonation is observed due to an increase in IHB strength and the A<sup>−</sup> anion additional stabilization. A proton donor ability comparative characteristic of the –SO<sub>3</sub>H group in the studied <i>ortho</i>-substituted benzenesulfonic acids is given, and the Δ<sub>r</sub>G<sup>0</sup><sub>298</sub> energies are compared with the corresponding values of <i>ortho</i>-substituted benzoic acids.
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