Estimation of thermodynamic and physicochemical properties of the alkali astatides: On the bond strength of molecular astatine (At2) and the hydration enthalpy of astatide (At−)

The recent accurate and precise determination of the electron affinity (EA) of the astatine atom At⁰ warrants a re-investigation of the estimated thermodynamic properties of At⁰ and astatine containing molecules as this EA was found to be much lower (by 0.4 eV) than previous estimated values. In this contribution we estimate, from available data sources, the following thermodynamic and physicochemical properties of the alkali astatides (MAt, M = Li, Na, K, Rb, Cs): their solid and gaseous heats of formation, lattice and gas-phase binding enthalpies, sublimation energies and melting temperatures. Gas-phase charge-transfer dissociation energies for the alkali astatides (the energy requirement for M⁺At⁻ ➔ M⁰ + At⁰) have been obtained and are compared with those for the other alkali halides. Use of Born-Haber cycles together with the new AE (At⁰) value allows the re-evaluation of ΔH_f (At⁰)_g (=56 ± 5 kJ/mol); it is concluded that (At₂)_g is a weakly bonded species (bond strength <50 kJ/mol), significantly weaker bonded than previously estimated (116 kJ/mol) and much weaker bonded than I₂ (148 kJ/mol), but in agreement with the finding from theory that spin-orbit coupling considerably reduces the bond strength in At₂. The hydration enthalpy (ΔH_aq) of At⁻ is estimated to be −230 ± 2 kJ/mol (using ΔH_aq[H⁺] = −1150.1 kJ/mol), in good agreement with molecular dynamics calculations. Arguments are presented that the largest alkali halide, CsAt, like the smallest, LiF, will be only sparingly soluble in water, following the generalization from hard/soft acid/base principles that “small likes small” and “large likes large.”