Extraction and DFT study on the complexation of H3O+ with hexaethyl p-tert-butylcalix[6]arene hexaacetate

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H3O+(aq) + 1· Na+(nb) ⇆ 1·H3O+(nb) + Na+(aq) taking place in the two-phase water–nitrobenzene system (1 = hexaethyl p-tert-butylcalix[6]arene hexaacetate; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (H3O+, 1·Na+) = −0.6 ± 0.1. Further, the stability constant of the 1·H3O+ complex in water saturated nitrobenzene was calculated for a temperature of 25 °C as log βnb (1·H3O+) = 6.8 ± 0. 2. By using quantum mechanical DFT calculations, the most probable structure of the 1·H3O+ complex species was predicted. In this complex, the hydroxonium ion H3O+ is bound partly to three carbonyl oxygen atoms by strong hydrogen bonds and partly to three alternate phenoxy oxygens by somewhat weaker hydrogen bonds.