Dispersion-corrected density functional theory (DFT-D), combined with the conductor-like polarizable continuum model (CPCM), was employed to investigate proton transfer processes in sulfonic acid-ammonia and alkylamine systems, including the formation of both ion pairs and free ions. Calculations were performed across a wide range of dielectric constants (from ε = 1.0 for the gas phase to ε = 78.36 for water). Proton transfer from the acid to the base occurs in all studied systems, except for the acid-ammonia pair in the gas phase, resulting in the formation of ion pairs regardless of ε. The probability of free ion formation also increases with rising ε. The structures of ion pairs formed by ammonium and alkyl-substituted ammonium cations (methyl-, dimethyl-, trimethyl-, triethyl-, and diethylmethylammonium) with mesylate, triflate, and hydrogen sulfate anions, along with their fully dissociated counterparts, were thoroughly analyzed. Special attention was given to hydrogen bonding interactions and their role in ion pair formation. Solvation energies were estimated and analyzed across the studied range of dielectric constants. The most pronounced changes in the calculated parameters were observed at ε < 20, with higher values producing only minor effects. The probable structures of the ammonium hydrogen sulfate cluster 2(NH4/HSO4) were also investigated. The presence of multiple hydrogen bonds, including both cation-anion and anion-anion interactions, was found to significantly enhance cluster stability.
