Hydrogen-bond effect and ion-pair association in the separation of neutral calix[4]pyrroles by nonaqueous capillary electrophoresis

A study on the migration behavior of four neutral macrocyclic compounds calix[4]pyrroles (C4Ps) by nonaqueous capillary electrophoresis (NACE) with tetrabutylammonium halide salts as background electrolyte (BGE) is described. Systematic studies were carried out with different BGE in acetonitrile (ACN) to elucidate the involved phenomena. The effect of BGE concentration on analytes effective mobilities μeff,i was studied. Rough values of analytes absolute mobilities μ∞,i were obtained by extrapolation of effective mobilities to zero ionic strength of BGE. Stokes radius of C4Ps in the form of C4PF− and C4PCl− was calculated. The complex constants of C4Ps with F− anion and Cl− anion and ion-pair association constants of each C4PF− and C4PCl− with tetrabutylammonium cations was evaluated and confronted. Hydrogen bonding effect is prerequisite in the separation; meanwhile ion-pair association which is intensified by steric hindrance effect plays an important role.