Precise relative proton affinities from a recent pulsed ion cyclotron resonance spectroscopy study have made it possible to evaluate the medium effects of water and of fluorosulfuric acid on the standard free energy and/or enthalpy change for reaction 1, B + NH4+ ⇋ BH+ + NH3, involving a wide variety of onium ions. The large medium effects (up to 50 kcal mol-1) are associated primarily with onium ion solvation, although effects of neutral bases of different types are not negligible. With the use of a thermodynamic cycle, evaluations have been made of the relative heats and free energies (water only) of transfer of onium ions from the gas phase to dilute solution in water and in fluorosulfuric acid. An analysis of the relative onium ion hydration entropies and energies has been carried out which approximately separates the contributions of hydrogen bonding (HB) terms from the sum of “physical” terms associated with differential cavity sizes, solvent structure, van der Waals dispersion forces, and electrostatic interactions. It is shown that these “physical” terms make relatively minor contributions to the differences in the heats of transfer from the gas to aqueous phase for onium ions which have the same hydrocarbon content and substitutional pattern at the central atom. The major contributions to such differential heats of transfer are the differences in the favorable HB terms arising from the association of water clusters with each of the available protonic sites of the onium ions. The energy differential for the HB terms is found to be approximately the same as the corresponding difference in binding energy of single water attachments in the gas phase. The magnitude of electrostatic hydration energy for the aquated ions, OH3+⋯[(OH2)n]3 and NH4+⋯[(OH2)n]4, has been estimated. There is a marked reduction in solution compared to the gas phase in the stabilization of ammonium and oxonium ions by polarization of hydrocarbon substituents. The strong H-bond donor protons of these ions give rise to ionic polarization of the H-bonded solvent in preference to ionic polarization of the hydrocarbon substituents. The relative free energies of transfer of substituted ammonium and oxonium ions from the gas phase to aqueous solution are shown to contain large unfavorable contributions from such electrostatic polarizability effects. The corresponding relative heats of transfer are much less dependent upon unfavorable polarizability effects because of compensating favorable effects of hydrocarbon substituents on solvent structure and van der Waals dispersion force terms. The relative heats of hydration of gaseous onium ions are determined primarily by favorable HB terms which depend upon the number and kind of acidic protons present. Certain substituents are shown to also exert dipolar electron-withdrawing effects, steric hindrance, and resonance effects on the HB solvation energy terms. In fluorosulfuric acid solutions, the relative heats of solvation of gaseous onium ions depend in particular on the number of protonic sites present, with much less dependence (compared to the results in water) on the type of protonic site. The strong leveling effect on HB terms in this solvent is especially evident in the appreciably smaller solvation energies of oxonium ions in fluorosulfuric acid as compared with aqueous solution. The relative heats of transfer from water to fluorosulfuric acid solutions for substituted ammonium and oxonium ions are found to be dominated by differential HB terms. These heats of transfer also are found to correlate with the Bunnett activity coefficient parameter, ¤>e, for indicator bases in H2SO4-H2O media. © 1978, American Chemical Society. All rights reserved.
