Experimental and theoretical study on complexation of the lithium cation with [Gly6]-antamanide

On the basis of extraction experiments and -activity measurements, the extraction constant corresponding to the equilibrium Li+(aq) + 1Na(+)(nb) 1Li(+) (nb) + Na+(aq) occurring in the two-phase water-nitrobenzene system (1= [Gly(6)]-antamanide; aq= aqueous phase, nb= nitrobenzene phase) was determined as logK(ex)(Li+,1Na(+))= 0.6 +/- 0.1. Further, the stability constant of the 1Li(+) complex in nitrobenzene saturated with water was calculated for a temperature of 25 degrees C: log(nb)(1Li(+))= 5.8 +/- 0.2. Finally, by using quantum chemical calculations, the most probable structure of the cationic complex species 1Li(+) was derived. In the resulting complex, the central' cation Li+ is bound by four bonding interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1Li(+) complex structure is stabilised by two intramolecular hydrogen bonds. The interaction energy of the considered 1Li(+) complex was found to be -553.5 kJ/mol, confirming also the formation of this cationic species. [GRAPHICS]