A computational study of the aqueous pertechnetate anion: Elucidation of the hydration structure and spectroscopic...

A computational study of the TcO₄^- anion in water has been carried out, the study includes an analysis of the structure of the solvation shells, the hydration energy and the electronic transitions that affect the shape of the UV-Vis absorption spectrum. The solvated system was characterized by combined QM/MM molecular dynamics simulations. The second-order perturbation theory restricted active space method and the novel self-consistent field density matrix renormalization group approach were employed to describe the static correlation. Two distinct solvation shells were found with 24 and 65 water molecules, respectively. The water molecules surrounding the anion have a general orientation in which one hydrogen is oriented away from the oxyanion while the remaining atoms are at a similar distance. However, there are specific water molecules that form hydrogen bonds with the oxygens of the oxyanion, while others are oriented with their oxygen atom towards the anion. The observed excitations in the UV-Vis spectrum are of a T₂ character, with the main source of the observed behavior being the charge transfer from oxygen atoms to the central technetium atom. The calculations show that the most intense band of the spectrum is broader and has a blue tail with respect to the gas phase spectrum. This difference is due to the lower symmetry caused by the aqueous environment, which allows different states to mix and leads to broadening of the band.