Ion model independent studies of hydrogen bonding and the thermochemistry of bifluoride salts

Abstract

Quantum mechanical based studies of the enthalpy change, ΔH^⊖₁, for the process (measuring the hydrogen bond strength in the bifluoride ion): HF^–₂(g)→ HF(g)+ F^–(g) are at variance with studies based on classical lattice energy calculations. One of the difficulties in the latter calculations has been that of making reliable calculations of the repulsion energy U_R between the non-spherical HF^–₂ ions, requiring the choice of a suitable model to represent the ion. The recently proposed equation: U_R=ρ/a[U_ELEC+ 6U_dd+ 8U_qd–K(∂M_ELEC/∂a)_{d, a=a0}] is used for ion model independent calculations of U_R. Incorporation of the results to calculate the lattice energy of alkali metal bifluoride salts and ΔH^⊖_f(HF^–₂)(g) leads to values for ΔH^⊖₁ which accord with the numerous quantum mechanical estimates. The results of the studies in this paper give, for the lattice potential energies of the bifluorides: U_POT(LiHF₂)= 893 ± 20 kJ mol^–1, U_POT(NaHF₂)= 788 ± 14 kJ mol^–1, U_POT(KHF₂)= 703 kJ mol^–1, U_POT(RbHF₂)= 674 ± 5 kJ mol^–1 and U_POT(CsHF₂)= 646 ± 4 kJ mol^–1, for ΔH^⊖_f(HF^–₂)(g) a value of –728 ± 9 kJ mol^–1 and for ΔH^⊖₁ a value = 176 ± 6 kJ mol^–1. We estimate U_POT(NH₄HF₂)= 705 ± 9 kJ mol^–1.