Nonaqueous capillary electrophoresis and quantum chemical calculations applied to investigation of acid-base and electromigration properties of azahelicenes

Nonaqueous capillary electrophoresis (NACE) using methanol (MeOH) as a solvent of the BGEs and quantum mechanical density functional theory (DFT) have been applied to determine the thermodynamic acidity (ionization) constants (pK(a)) of mono- and diaza[5]helicenes, mono- and diaza[6]helicenes, and their dibenzo derivatives in MeOH and water. First, the mixed acidity constants, pKa,MeOHmix, of ionogenic pyridinium groups of azahelicenes and their derivatives in MeOH were obtained by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by NACE in a large series of methanolic BGEs within a wide conventional pH range (pH(MeOH) 1.6-12.0) and at ambient temperature (21-26 degrees C) in a home-made CE device. Prior to mixed acidity constant calculation, the effective mobilities were corrected to reference temperature (25 degrees C) and constant ionic strength (25 mM). Then, the mixed acidity constants were recalculated to the thermodynamic acidity constants pK(a,MeOH) by the Debye-Huckel theory of nonideality of electrolyte solutions. Finally, from the methanolic thermodynamic pK(a,MeOH) values, the aqueous thermodynamic pKa,H2O constants were estimated using the empirical relations between methanolic and aqueous acidity constants derived for structurally related pyridine derivatives. Depending on the number and position of the nitrogen atoms in their molecules, the analyzed azahelicenes were found to be weak to moderate bases with methanolic pK(a,MeOH) in the range 2.01-8.75 and with aqueous pKa,H2O in the range 1.67-8.28. The thermodynamic pK(a,MeOH) obtained by the DFT calculations were in a good agreement with those determined experimentally by NACE.