Gas Phase Solvation of the Ammonium Ion by NH3 and H2O and Stabilities of Mixed Clusters NH4+ (NH3)n(H2O)w

Abstract

The gas phase equilibria and those leading to mixed clusters like were measured with a pulsed electron beam high pressure ion source mass spectrometer. The ion source contained pure ammonia or mixtures of ammonia and water vapor at pressures in the Torr range. Determination of the temperature dependence of the equilibrium constants led to the evaluation of ΔG⁰, ΔH⁰, and ΔS⁰ values for the equilibria from n = 1 to 4 and w = 1 to 5. The ΔG⁰ values for the NH₄⁺(NH₃)_n equilibria were in good agreement with previous determinations from this laboratory. Fair agreement was observed for the ΔH⁰ and ΔS⁰ values. Comparison with the corresponding results for NH₄⁺(H₂O)_w showed that the ΔH_n,n−1 and ΔG⁰_n,n−1 were larger than ΔH_w,w−1 and ΔG⁰_w,w−1. The difference was largest for the first step (1,0) and decreased progressively until a reversal with water values becoming larger occurred at the (5,4) step. The stronger hydrogen bonds of NH₃ to NH₄⁺ for low ligand numbers is explained by the greater basicity of NH₃. As the ionic charge becomes dispersed and more distant stronger interactions are obtained with water which gives stronger H bonds in the absence of positive ionic charge. Breaks in the ΔH_n,n−1 values indicate existence of a relatively stable NH₄⁺(NH₃)₄ symmetric ion. A much smaller and less distinct break is observed with pure water ligands.The mixed ammonia–water clusters show similar effects. The addition of a NH₃ molecule to a pure water ligand complex gives the strongest interaction. The addition of H₂O to a pure ammonia cluster gives the weakest interaction. The above effect is strongest at lowest ligand numbers. The difference decreases gradually and becomes reversed for more than four ligands.