The Heaviest Possible Ternary Trihalogen Species, IAtBr- , Evidenced in Aqueous Solution: An Experimental Performance Driven by Computations

Abstract

Evidencing new chemical species in solution is particularly challenging when one works at ultra-trace concentrations, as is likely to happen with radioelements such as astatine (Z=85). Herein, quantum mechanical calculations were used to predict the narrow experimental domain in which it is possible to detect the presence of an exotic ternary trihalogen anion, IAtBr^- , and thus to guide a series of experiments. By analyzing the outcomes of competition experiments, we show that IAtBr^- exists and can even predominate in aqueous solution. The equilibrium constant associated with the reaction At⁺ +I^- +Br^- ⇌IAtBr^- was determined to be 10^7.5±0.2 , which is in fair agreement with that predicted by density functional theory (10^6.9 ). This system not only constitutes the very first example of a ternary trihalogen species that involves the element astatine but is also the first trihalogen species reported to predominate in solution. Moreover, we show that the oxidation number of At is zero in this species, as in the other molecules and anions that At⁺ can form with Cl^- , Br^- , and I^- ligands.

Keywords: astatine; competition experiments; density functional calculations; halogens; ternary trihalogen species.