Julie Champion

Astatine Standard Redox Potentials and Speciation in Acidic Medium

A combined experimental and theoretical approach is used to define astatine (At) speciation in acidic aqueous solution and to answer the two main questions raised from literature data: does At(0) exist in aqueous solution and what is the chemical form of At(+III), if it exists. The experimental approach considers that a given species is characterized by its distribution coefficient (D) experimentally determined in a biphasic system. The change in speciation arising from a change in experimental conditions is observed by a change in D value. The theoretical approach involves quasi-relativistic quantum chemistry calculations. The results show that At at the oxidation state 0 cannot exist in aqueous solution. The three oxidation states present in the range of water stability are At(-I), At(+I), and At(+III) and exist as At(-), At(+), and AtO(+), respectively, in the 1-2 pH range. The standard redox potentials of the At(+)/At(-) and AtO(+)/At(+) couples have been determined, the respective values being 0.36 +/- 0.01 and 0.74 +/- 0.01 V vs NHE.

Investigation of astatine(III) hydrolyzed species: experiments and relativistic calculations.

This work aims to resolve some controversies about astatine(III) hydroxide species present in oxidant aqueous solution. AtO(+) is the dominant species existing under oxidizing and acidic pH conditions. This is consistent with high-performance ion-exchange chromatography data showing the existence of one species holding one positive charge. A change in speciation occurs as the pH changes from 1 to 4, while remaining under oxidizing conditions. Dynamic experiments with ion-exchange resins evidence the existence of a neutral species witnessed by its elution in the void volume. Batch-experiments using a competition method show the exchange of one proton indicating the formation of the AtO(OH) species. The hydrolysis thermodynamic constant, extrapolated to zero ionic strength, was determined to be 10(-1.9). This value is supported by two-component relativistic quantum calculations and therefore allows disclosing unambiguously the structure of the formed species.

Advances on the Determination of the Astatine Pourbaix Diagram: Predomination of AtO(OH)2− over At− in Basic Conditions

It is generally assumed that astatide (At(-) ) is the predominant astatine species in basic aqueous media. This assumption is questioned in non-complexing and non-reductive aqueous solutions by means of high-pressure anion-exchange chromatography. Contrary to what is usually believed, astatide is found to be a minor species at pH=11. A different species, which also bears a single negative charge, becomes predominant when the pH is increased beyond 7. Using competition experiments, an equilibrium constant value of 10(-6.9) has been determined for the formation of this species from AtO(OH) with the exchange of one proton. The identification of this species, AtO(OH)2 (-) , is achieved through relativistic quantum mechanical calculations, which rule out the significant formation of the AtO2 (-) species, while leading to a hydrolysis constant of AtO(OH) in excellent agreement with experiment when the AtO(OH)2 (-) species is considered. Beyond the completion of the Pourbaix diagram of astatine, this new information is of interest for the development of (211) At radiolabeling protocols.

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

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 107.5±0.2 , which is in fair agreement with that predicted by density functional theory (106.9 ). 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.

Experimental and computational evidence of halogen bonds involving astatine

The importance of halogen bonds—highly directional interactions between an electron-deficient σ-hole moiety in a halogenated compound and an acceptor such as a Lewis base—is being increasingly recognized in a wide variety of fields from biomedicinal chemistry to materials science. The heaviest halogens are known to form stronger halogen bonds, implying that if this trend continues down the periodic table, astatine should exhibit the highest halogen-bond donating ability. This may be mitigated, however, by the relativistic effects undergone by heavy elements, as illustrated by the metallic character of astatine. Here, the occurrence of halogen-bonding interactions involving astatine is experimentally evidenced. The complexation constants of astatine monoiodide with a series of organic ligands in cyclohexane solution were derived from distribution coefficient measurements and supported by relativistic quantum mechanical calculations. Taken together, the results show that astatine indeed behaves as a halogen-bond donor—a stronger one than iodine—owing to its much more electrophilic σ-hole.Halogen bonding is known to get stronger with increasing halogen polarizability, but some trends of the periodic table break down for heavy elements owing to relativistic effects. Now, through distribution coefficient measurements and relativistic quantum mechanical calculations, AtI has been shown to form stronger halogen bonds than I2—meaning that astatine conforms to the trend.

A route for polonium 210 production from alpha-particle irradiated bismuth-209 target

Abstract A method is proposed for production of polonium-210 via the 209Bi(α, 3n)210At nuclear reaction. Bombardment of a bismuth-209 target was performed with a 37 MeV alpha-particle beam that leads to the production of astatine-210 (T1/2 = 8.1 h), which decays to polonium-210. It is purified from the bismuth target matrix by employing liquid–liquid extraction using tributyl phosphate (TBP) in para-xylene from 7 M hydrochloric acid. Back extraction of polonium-210 was performed by 9 M nitric acid. This method allows to purify a tracer amount of Po-210 (2.6 · 10–13 mol) from macroscopic amount of Bi (2.8 · 10–2 mol).

Evidence for the Heaviest Expected Halide Species in Aqueous Solution, At–, by Electromobility Measurements

At- (astatide) is commonly expected to be the heaviest halide in the halogen group. However, there is no proof for the existence of this -1 charged species. Furthermore, investigations with astatine are restricted by its specific radioactive properties, which entail working at ultratrace concentrations (typically less than 10-10 M). In this work, an especially built electromigration device is applied to obtain information about the charge/size ratio characterizing an ion in aqueous solution. An anionic At species is observed in reducing conditions. Moreover, we propose the first absolute mobility value for the astatine species in acidic reducing condition: (-8.26 ± 0.59) × 10-4 cm2·V-1·s-1. This value appears close to that of I- ((-8.30 ± 0.33) × 10-4 cm2·V-1·s-1), which is obtained by the same method. The similar absolute mobilities obtained for both ions are coherent with theoretical calculations indicating similar diffusion behaviors for At- and I-. This good agreement confirms the existence of the At- species.