Hydrogen Bond Strength and Charge Distribution in the Bifluoride Ion, HF2−. Lattice Energies of LiHF2, NaHF2, KHF2, RbHF2, and CsHF2

Abstract

Electrostatic energies and Madelung parameters for LiHF2, NaHF2, KHF2, RbHF2, and CsHF2 are reported together with a term-by-term calculation of the total cohesive lattice energy, all expressed as single-parameter quadratic functions of the charge Q(F) on the terminal fluorine atoms of the linear HF2− ion. An assessment of the charge distribution in this ion is made in conjunction with a calculation of the enthalpy change, Δ H1, of the reaction, HF2−(g)→ HF(g) + F−(g),thus measuring the strength of the hydrogen bond. The intersections of the curves for Δ H1 as a function of Q(F) are used to assign values of −0.73 e.u. to Q(F) and 252 kJ mol−1 to Δ H1 and the following values for the total potential lattice energies: Epot(LiHF2)=820.6 kJ mol−1; Epot(NaHF2)=755.3 kJ mol−1; Epot(KHF2)=648.2 kJ mol−1; Epot(RbHF2)=626.9 kJ mol−1, and Epot(CsHF2)=607.4 kJ mol−1.