Kinetics and Thermodynamics of Ion Pair Dissociation to Yield Free Solvated Ions. Effect of Steric Hindrance

Article abstract of DOI:10.1021/j100444a018

The kinetics and thermodynamics of ion pair dissociation involving the anion radicals of ninhydrin, nitrobenzene, and trialkyl-substituted nitrobenzenes ion associated with Na⁺, K⁺, Cs⁺, or (Bu)4N⁺ in hexamethylphosphoramide (HMPA) were studied via ESR spectroscopy.The large alkyl groups on the (Bu)4N⁺ cation were found to sterically inhibit the close approach of the positive nitrogen to the anion radical.However, this loose ion pair dissociates less exothermically and exoenergetically than tighter ion pairs involving the other cations due to the poorer solvation of this large cation by the HMPA.The alkyl groups in the ortho positions of the nitrobenzene anion radical also prevent close approach of a solvated cation to yield intimate ion pairs.These loose ion pairs have lower free energies of activation for ion pair disociation.The observed rates and thermodynamics of ion pair dissociation and formation are explained in terms of ion solvation and steric inhibition to ion association.