Ion-pair dissociation equilibria for trityl hexafluoro-arsenate and -antimonate in polar solvents

Abstract

Dissociation constants (K_D) for Ph₃C⁺AsF₆^– and Ph₃C⁺SbF₆^– in CH₂Cl₂ and CH₂Cl₂–CH₃NO₂ were determined. The entropies and enthalpies of dissociation in CH₂Cl₂(AsF₆^–ΔH–2.2 ± 0.2 kcal mol^–1, ΔS–25.1 ± 0.8 cal mol^–1 K^–1, SbF₆–ΔH–2.2 ± 0.5 kcal mol^–1, ΔS–25 ± 2 cal mol^–1 K^–1) are in agreement with the previously determined parameters for SbCl₆– anion, taking into account the larger diameter of the latter anion. The values of the Stokes radii of AsF₆– and SbF₆– ions (both 2.2 Å) as well as the effective interionic distances in both ion-pairs (equal to 7.1 Å) were evaluated. The values of K_D increase ca. 10² times at 25 °C on passing from CH₂Cl₂ to CH₃NO₂ as solvent. This gives an opportunity to study reactions with predominantly paired (in CH₂Cl₂) or free (in CH₃NO₂) cations.