Ab Initio Speciation of Tc-Gluconate Complexes in Aqueous Systems

Tc-gluconate complexes in aqueous systems were recently reported and characterized by Tc L₃-edge X-ray absorption near-edge structure (XANES) measurements [Dardenne, K.; Inorg. Chem. 2021, 60, 12285-12298]. The puzzling result was reported that the Tc L₃-edge XANES of the sample containing Tc(IV)-gluconate species differs substantially from that of the Tc(IV)O₂(am,hyd) hydrous oxide reference sample, whereas the Tc K-edge XANES spectra did not differ significantly. We studied this observation theoretically and tracked the unknown Tc(IV)-gluconate species in a three-step procedure: (1) developing chemical models, (2) optimizing the equilibrium structures of the models, and (3) simulating the corresponding Tc L₃-edge XANES spectra. We identified the [Tc(IV)(Glu_-2H)₂(H₂O)₂]^2- structure as the most likely Tc(IV)-gluconate species present in our samples and explain the substantial difference between the two Tc L₃-edge XANES spectra. Additionally, we revisited the Tc(V)-gluconate species and identified the [Tc(V)O(Glu_-H)₂]^- structure as the most likely Tc(V)-gluconate species in our sample.