Dielectric increments, intercharge distances and conformation of quaternary ammonioalkylsulfonates and alkoxydicyanoethenolates in aqueous and tirfluoroethanol solutions

Abstract

ω-(Triethylammonio)alkane-α-sulfonates (H₅C₂)₃N⁺–(CH₂)_n–SO₃^–(n= 2–4), pyridinio-, 2,6-lutidinio- and N-methylimidazolinio-propanesulfonates, and triethylammonioalkoxydicyanoethenolates (H₅C₂)₃N⁺–(CH₂)_n–O–CO–C^–(CN)₂(n= 2, 3) have been studied by dielectric measurements (1 MHz–1 GHz) in dilute aqueous or trifluoroethanol solution. Their intercharge distances have been derived from their dipole moments (µ in the range 18–30 D) calculated according to Buckingham's theory, and compared with the maximum and minimum intercharge distances estimated by molecular mechanics for the ideal fully extended and ion-pair conformations respectively. For identical tether length, stronger charge delocalization in the cationic (imidazolinium versus pyridinium) or anionic (dicyanoethenolate versus sulfonate) sites slightly increases the intercharge distance, while better specific solvation of the zwitterions by hydrogen bonding in the less dissociating trifluoroethanol does not result in any significant conformational change with respect to aqueous solutions. Folding of the intercharge arm, negligible for 2-(triethylammonio)ethane-1 -sulfonate, progressively increases with its length, but the ion-pair conformation, where the intercharge distance becomes rapidly insensitive to the tether length, is never reached. This behaviour is in good agreement with the variations of the apparent ΔpK_a values of the zwitterions (normalized to that of n-C₄H₉SO₃H) measured by potentiometry in acetic anhydride-acetic acid (9:1, v/v) solution.