Application of affinity capillary electrophoresis and density functional theory to the investigation of benzo-18-crown-6-ether complex with ammonium cation

Affinity capillary electrophoresis (ACE) and quantum mechanical density functional theory (DFT) calculations have been employed for investigation of non-covalent interactions between macrocyclic ligand, benzo-18-crown-6-ether (B18C6) and ammonium cation, NH4+. Firstly, by means of ACE, the strength of the B18C6-NH4+ complex in mixed binary hydro-organic solvent system, methanol–water (50/50, v/v), was determined from the dependence of effective electrophoretic mobility of B18C6 (corrected to reference temperature 25 °C and constant ionic strength, 10 mM) on the concentration of ammonium ion in the background electrolyte (BGE) using non-linear regression analysis. The logarithmic form of the apparent binding (stability) constant (log Kb) of B18C6-NH4+ complex in the above binary solvent system was found to be equal to log Kb = 1.63 ± 0.10. Secondly, the structural characteristics of B18C6-NH4+ complex were described by quantum mechanical density functional theory (DFT) calculations. According to these calculations, in the energetically most favoured form of the B18C6-NH4+ complex, three strong hydrogen bonds are formed between central ammonium ion and B18C6 ligand, one of them is directed to aryl-O-alkyl (Ar–O–CH2) ethereal oxygen and the other two hydrogen bonds are oriented to alkyl-O-alkyl (CH2–O–CH2) ethereal oxygen atoms of the macrocyclic crown ligand.