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

On the basis of extraction experiments and gamma-activity measurements, the extraction constant corresponding to the equilibrium K+ (aq) + 1 center dot Na+ (nb) reversible arrow 1 center dot 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 K-ex (K+, 1 center dot Na+) = 0.2 +/- 0.1. Further, the stability constant of the 1 center dot K+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 degrees C: log beta(nb) (1 center dot K+) = 4.7 +/- 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1 center dot 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 center dot K+ complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1 center dot K+ complex was found to be -346.6 kJ/mol, confirming also the formation of this cationic species. (C) 2014 Elsevier B.V. All rights reserved.