Protonation of sodium ionophore III: Extraction and DFT study

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H3O+(aq) + 1⋅Na+(nb) ⇆ 1⋅H3O+(nb) + Na+(aq) occurring in the two-phase water–nitrobenzene system (1 = sodium ionophore III; aq = aqueous phase, nb = nitrobenzene phase) was determined as log Kex (H3O+, 1⋅Na+) = 0.0 ± 0.1. Further, the stability constant of the 1⋅H3O+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb (1⋅H3O+) = 6.4 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1⋅H3O+ was derived. In the resulting complex, the “central” cation H3O+ is bound by two linear hydrogen bonds and one bifurcated hydrogen bond to the corresponding four oxygens of the parent ligand 1. The interaction energy of the considered 1⋅H3O+ complex was found to be −556.6 kJ/mol, confirming also the formation of this cationic species.