A correlation of the energy barrier opposing cation reorientation in ammonium compounds with thermochemical data

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–)={ΔH_f^⊖[(NH₄)_nX,s]–ΔH_f^⊖(Rb_nX,s)}/n, and at zero barrier height, Δ(X^n–) is about 143 kJ mol^–1; depressions below this threshold provide some measure of cation–anion hydrogen bonding in (NH₄)_nX, and the estimated value of ΔH_f^⊖(NH₄⁺,g), derived from the threshold, is 638 kJ mol^–1.