Solvation energies of ions and ionic transition states from studies of gas-phase ion–molecule reactions and equilibria

Abstract

Gas-phase ion–molecule reaction equilibrium measurements of reactions such as proton transfer and electron transfer have provided gas-phase thermochemical data for thousands of ions which are of importance also in solution. The solvation energies of such ions can be evaluated via Born-type cycles provided that the supporting thermochemical data for the liquid solvents are available. The resulting solvation enthalpies and free energies can be analysed on the basis of gas-phase data providing the sequential energy release in the initial solvation of the ion M^± by solvent molecules S₁ in the gas phase: M^±(Sl)_n–1+ Sl = M^±(Sl)_n, where the ΔG^°_n–1,n and ΔH^°_n–1,n are obtained from measurements of the gas-phase equilibria. As examples of this approach the hydration of NH⁺₄ and H₃O⁺ are examined. Data for the solvation of radical anions of substituted benzenes and quinones are given also. Kinetic studies of gas-phase ion–molecule reactions provide energetics for the key points of the reaction coordinate. Comparisons of the reaction coordinates in the gas phase and solution lead, via Born cycles, to solvation energies of the transition states. Data are provided for the nucleophilic displacement reactions: Cl^–+ CH₃Br →(ClCH₃Br)^–→ ClCH₃+ Br^– and F^–+ C₆F₆→ C₆F^–₇→ C₆F₆+ F^–.