Dimensions of tetra-alkyl(aryl)onium ions

Abstract

When considering the dimensions of tetra-alkyl(aryl)onium ions it is important to take into consideration the medium in which these ions are situated, since the behaviour of the organic chains of these ions depends on their surroundings. The Stokes radii for such large ions in organic solvents are considered to be the real dimensions of these ions in their unsolvated form. The calculated Stokes radii for tetra-alkylammonium cations, beginning with the Pr₄N⁺ ion, and for some other large tetra-alkyl(aryl)onium ions are virtually independent of the nature of the organic solvent. However, these radii differ from the experimental values in H₂O and D₂O, where the specific hydrophobic hydration phenomenon associated with the formation of clathrate-like structures occurs. The virtual independence from organic solvent of the solvation radii of large tetra-alkyl(aryl)onium ions is a striking demonstration that these ions are unsolvated in organic solvents. The radii of NH⁺₄ and Me₄N⁺ ions were calculated using a structural model.