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

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 ${\mathrm{HF}}_2^{-}$ ion. An assessment of the charge distribution in this ion is made in conjunction with a calculation of the enthalpy change, ${\mathrm{\Delta \; H}}_1$ , of the reaction, $${\mathrm{HF}}_2^{-}(\mathrm{g}\mathrm{)\to }\mathrm{HF}(\mathrm{g}\mathrm{)\; +}{\mathrm{F}}^{-}(\mathrm{g})$$ , thus measuring the strength of the hydrogen bond. The intersections of the curves for ${\mathrm{\Delta \; H}}_1$ 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 ${\mathrm{\Delta \; H}}_1$ and the following values for the total potential lattice energies: $E_{\mathrm{pot}}({\mathrm{LiHF}}_2\text{)=820.6 \hspace{0.17em}}\mathrm{kJ}\hspace{0.17em}{\mathrm{mol}}^{\text{−1}}{\mathrm{;\; E}}_{\mathrm{pot}}({\mathrm{NaHF}}_2\text{)=755.3\hspace{0.17em}}\mathrm{kJ}\hspace{0.17em}{\mathrm{mol}}^{\text{−1}}{\mathrm{;\; E}}_{\mathrm{pot}}({\mathrm{KHF}}_2\text{)=648.2\hspace{0.17em}}\mathrm{kJ}\hspace{0.17em}{\mathrm{mol}}^{\text{−1}}{\mathrm{;\; E}}_{\mathrm{pot}}({\mathrm{RbHF}}_2\text{)=626.9\hspace{0.17em}}\mathrm{kJ}\hspace{0.17em}{\mathrm{mol}}^{\text{−1}},\mathrm{and}{\mathrm{\hspace{0.17em}E}}_{\mathrm{pot}}({\mathrm{CsHF}}_2\text{)=607.4\hspace{0.17em}}\mathrm{kJ}\hspace{0.17em}{\mathrm{mol}}^{\text{−1}}$ .