Experimental and DFT study on the complexation of NH4+ with a hexaarylbenzene-based receptor

From extraction experiments and gamma-activity measurements, the extraction constant corresponding to the equilibrium NH4+(aq) + 1 center dot Cs+ (nb) reversible arrow 1 center dot NH4+(nb) + Cs+(aq) taking part in the two-phase water-nitrobenzene system (1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K-ex(NH4+, 1 center dot Cs+) = -0.4 +/- 0.1. Further, the stability constant of the 1 center dot NH4+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 circle C as log beta(nb)(1 center dot NH4+) = 6.3 +/- 0.2. By using quantum mechanical OFT calculations, the most probable structures A and B of the 1 center dot NH4+, complex species, which are obviously in a dynamic equilibrium, were indicated. In both of these structures of the resulting complex 1 center dot NH4+ having C-3 symmetry, the cation NH4+ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene bottom via cation-pi interaction. (C) 2010 Elsevier B.V. All rights reserved.