Experimental and theoretical study on interaction of the silver cation with nonactin

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium occurring in the two-phase water–nitrobenzene system (1 = nonactin, aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K_ex (Ag⁺, 1 ⋅ Na⁺) = 0.6 ± 0.1. Furthermore, the stability constant of the 1 ⋅ Ag⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β_nb(1 ⋅ Ag⁺) = 6.6 ± 0.2. Finally, employing quantum mechanical calculations, the most probable structure of the cationic complex species 1 ⋅ Ag⁺ was derived. In the resulting complex, having a tennis-ball-seam conformation with the C₂ symmetry, the ‘central’ cation Ag⁺ is bound by eight relatively strong bonding interactions to eight oxygen atoms of the parent nonactin ligand. The interaction energy of the considered 1 ⋅ Ag⁺ complex was found to be –468.5 kJ/mol, confirming also the formation of this cationic species.