Binding of water in aqua 18-crown-6 dichloropicric acid complexes in the solid state and its reversible thermal removal

Abstract

Thermogravimetry, differential scanning calorimetry, and FT-IR vibrational spectral studies on the 1:1:1 18-crown-6: dichloropicric acid: water complex reveal that it remains unaltered when heated to melting. On the other hand, one molecule of water is expelled from the 1:2:2 complex by heating prior to melting. This results in an ionic microcrystalline product of undetermined crystal structure, in which the conformation of the crown is lower than D3d in symmetry. The enthalpy accompanying this process, which involves rupturing of two water–crown hydrogen bonds, is −44 to −55 kJ mol−1. The original 1:2:2 complex is regenerated by exposure of the heated adduct to water vapor. It was found that H2O in the structure of the solid 1:2:2 adduct was partially replaced by D2O at room temperature upon exposure to D2O vapor. The crown–H2O OH stretch in the IR exhibits a small or negligible positive isotope effect on deuteration. Keywords: crown ether, hydrogen bonding, water, dichloropicric acid.