The thermochemistry of ammonium compounds and the energy barrier opposing ammonium ion reorientation

Abstract

There is a good linear correlation between the energy barrier opposing cation reorientation in a compound (NH₄)_nX and the quantity Δ(X^n–)=1//_n{ΔH^⊖_f[(NH₄)_nX, s]–ΔH^⊖_f(Rb_nX, s)}. At zero barrier height, Δ(X^n–)= 143 ± 3 kJ mol^–1. There is a sense in which depressions below this threshold are a measure of cation–anion hydrogen bonding but this sense is an unconventional one. The estimated value of ΔH^⊖_f(NH₄⁺, g) derived from the threshold is 638 ± 9 kJ mol^–1. The correlation is also used to improve literature thermodynamic data on ammonium and alkali-metal perrhenates, e.g., ΔH^⊖_f(NH₄ReO₄, s)=–965 ± 5 kJ mol^–1. Unknown enthalpies of formation of some ammonium and rubidium salts are also predicted, and the procedure is used to explore the thermodynamic stability of non-existent solid ammonium compounds. Both metallic ammonium and ammonium hydroxide are unstable at normal pressures. The former is most unlikely to be stabilized at high pressure, but it may be possible to synthesize the hydroxide in this way. The problem of the ionic radius of NH₄⁺ is also discussed.