Complexation of the ammonium cation with dibenzo-18-crown-6: extraction and DFT study.

From extraction experiments and gamma-activity measurements, the extraction constan corresponding to the equilibrium NH4(+)(aq) + 1*Na(+)(nb) <=> 1*NH4(+)(nb) + Na(+)(aq) taking place in the two-phase water - nitrobenzene system (1 = dibenzo-18-crown-6, aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K(ex) (NH4(+),1*Na(+)) = -0.1 +/- 0.1. Further, the stability constant of the 1*NH4(+) complex species in water-saturated nitrobenzene was calculated for a temperature 25 degrees C as log beta (1*NH4(+)) = 5.7 +/- 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the 1*NH4(+) cationic complex was derived. In this complex, the "central" cation NH4(+) is bound by three strong linear hydrogen bonds to the three corresponding ethereal oxygen atoms of the parent crown ligand 1. The interaction energy of the resulting complex 1*NH4(+) was found to be -796.1 kJ/mol, confirming the formation of the considered complex species.