Structure of Protonated Forms of Cryptand[2.2.2] and Constants of Their Formation in Aqueous Solutions of Dimethyl Sulfoxide

Constants of protonation of cryptand[2.2.2] in aqueous solutions of dimethyl sulfoxide are determined via potentiometric titration at T = 298 K. It is observed that the constants of equilibrium of the formation of mono- and bi-protonated cryptand[2.2.2] are lower at weak concentrations of the non-aqueous component in the solvent than in an aqueous solution. Increasing the content of dimethyl sulfoxide raises the constants of the considered processes. No decisive contributions from solvation to the change in the constants of protonation are found in evaluating the effect the resolvation of reagents in an aqueous dimethyl sulfoxide has on the change in the Gibbs energy of the given reactions. Results from quantum chemical calculations of the structure of cryptand[2.2.2] and its protonated species show that the formation of a monoprotonated macrocycle is accompanied by compression of the cavity in the direction of the nitrogen-nitrogen axis, while the formation of a biprotonated macrocycle is in contrast accompanied by expansion of the cavity.