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 UR between the non-spherical HF–2 ions, requiring the choice of a suitable model to represent the ion. The recently proposed equation: UR=ρ/a[UELEC+ 6Udd+ 8Uqd–K(∂MELEC/∂a)d, a=a0] is used for ion model independent calculations of UR. 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: UPOT(LiHF2)= 893 ± 20 kJ mol–1, UPOT(NaHF2)= 788 ± 14 kJ mol–1, UPOT(KHF2)= 703 kJ mol–1, UPOT(RbHF2)= 674 ± 5 kJ mol–1 and UPOT(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 UPOT(NH4HF2)= 705 ± 9 kJ mol–1.