Author:

Keywords:

technetium, DFT, EXAFS - XANES, structure elucidation, Science & Technology, Physical Sciences, Chemistry, Inorganic & Nuclear, Chemistry, RAY-ABSORPTION SPECTROSCOPY, MOLECULAR-STRUCTURE, HUMIC SUBSTANCES, TECHNETIUM, COMPLEXES, TRIS(BETA-DIKETONATO)TECHNETIUM(III), TRIS(ACETYLACETONATO)TECHNETIUM(III), Models, Molecular, Molecular Conformation, Oxygen, Quantum Theory, Solvents, Technetium, X-Ray Absorption Spectroscopy, 0302 Inorganic Chemistry, 0307 Theoretical and Computational Chemistry, 0399 Other Chemical Sciences, Inorganic & Nuclear Chemistry, 3402 Inorganic chemistry

Abstract:

Technetium, an unusual element whereof only radioactive isotopes exist, is characterised by a very rich redox (-1 to +7) and coordination chemistry. Due to its importance for nuclear medicine and the considerable amount of Tc present in high level radioactive waste as a fission product from U it has been the focus of many studies for several decades.1-5 In absence of stabilising ligands Tc(VII) and Tc(IV) are the main, stable oxidation states in oxygenated and reducing conditions, respectively. In spite of the research efforts invested in this element, some important gaps in the basic Tc chemistry remain. The pH dependent speciation of dissolved Tc(IV) in aqueous solutions is thermodynamically described by the series TcO2+, TcOOH+, TcO(OH)2 and TcO(OH)3-. Nevertheless, the molecular structure of these Tc(IV) hydrolysis species is uncertain because of the difficulties to distinguish between oxy- and hydroxy- coordination in these sparingly soluble Tc(IV) species (solubility of TcO2.xH2O = 3x10-8 M). The description of hydrolyzed Tc(IV) species as TcO(OH)n2−n(aq) rather than Tc(OH)m4−m(aq) was mainly due to the observation that the charge of Tc(IV) species never exceeded +2 even at pH = 0.6 To our knowledge the existence of the TcO2+ core has not been detected spectroscopically in either hydrolyzed Tc(IV) species or in any other Tc(IV) coordination compound. In this contribution, XAS evidence for its existence will be provided using a strategy based on the combined use of X-ray Absorption Spectroscopy (XAS) and Density Functional methods (DFT). Using this methodology, detailed structural information can be obtained for non-crystalline and crystalline components in virtually all experimental conditions.