Thermochemical aspects of the rotational dynamics of the ammonium ion in an environment of water molecules

Abstract

The inverse correlation between the quantity 1/n{Δ_fH_m^θ[(NH₄)_nX,c]−Δ_fH_m^θ(Rb_nX,c)} and the barrier to cation reorientation in the ammonium compound is used to assess the connection between the strength of the hydrogen bonding and the rotational barrier of the ammonium ion in an environment of water molecules. This assessment required the value Δ_fH_m^θ[RbAl(SO₄)₂·12H₂O,c]=−6074.8±1.7 kJ mol⁻¹, which was derived by determining the difference of 132.4±1.5 kJ mol⁻¹ between the standard enthalpies of formation of ammonium and rubidium alum in a solution calorimeter. In the ammonium alum, the correlation is obeyed, the barrier being small and the hydrogen bonding weak. In aqueous solution, the correlation is not obeyed, the barrier being small and the hydrogen bonding substantial. The hydrogen bonding in the alum is weaker because the coordination in the alum does not permit formation of the four linear or nearly linear N–H···O hydrogen bonds which feature in most theoretically optimized structures of [NH₄(H₂O)_n]⁺ clusters and the aqueous ion. In aqueous solution, the barrier is only 50% of the value expected from the correlation. This is consistent with the recently proposed rotational mechanism in which two of the four N–H···O hydrogen bonds are preserved during reorientation.