Experimental and theoretical study on interaction of the potassium cation with antamanide

- Stability of the antamanide - K+ complex was determined.
- Quantum mechanical DFT calculations were applied.
- The structure of the resulting complex was predicted.

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium K+ (aq) + 1·Na+ (nb) ⇄1·K+ (nb) + Na+ (aq) taking part in the two-phase water-nitrobenzene system (1 = antamanide; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (K+, 1 · Na+) = 0.2 ± 0.1. Further, the stability constant of the 1 · K+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb (1 · K+) = 4.7 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1 · K+ was derived. In the resulting complex, the “central” cation K+ is bound by four bond interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1 · K+ complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1 · K+ complex was found to be −346.6 kJ/mol, confirming also the formation of this cationic species.