Extraction and DFT study on complexation of the barium cation with [Gly6]-antamanide

Abstract

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Ba2+(aq) + 1·Sr2+(nb) ⇄1·Ba2+ (nb) + Sr2+(aq) occurring in the two–phase water – nitrobenzene system (1 = [Gly6]-antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log Kex (Ba2+,1·Sr2+) = 0.6 ± 0.1. Further, the stability constant of the 1·Ba2+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb (1·Ba2+) = 9.5 ± 0.2. Finally, by using quantum chemical DFT calculations, the most probable structure of the cationic complex species 1·Ba2+ was derived. In the resulting complex, the “central” cation Ba2+ is bound by six bonding interactions to the corresponding six carbonyl oxygen atoms of the parent ligand 1. The interaction energy of the considered 1·Ba2+ complex was found to be −943.5 kJ/mol, confirming the formation of this cationic species as well