UV-Vis Absorption Spectroscopy of Polonium(IV) Chloride Complexes: An Electronic Structure Theory Study

More than a hundred years after its discovery, the chemistry of the polonium radioelement is still largely unknown. However, it is quite clear that the properties of this heavy element ( Z = 84) may be affected by relativistic effects, in particular scalar relativistic effects and the so-called spin-orbit coupling (SOC). In this Article, we revisit the interpretation of UV-vis absorption spectra of polonium(IV) complexes in HCl medium, reported decades ago. From the data, two complexes were hypothesized, complex I with a maximum of absorption at 344 nm (at low HCl concentration) and complex II with a maximum at 418 nm (the only visible peak for HCl concentrations above 0.5 M). By monitoring the absorbance at 344 and 418 nm as a function of both the HCl concentration and the pH, complex I was formulated as [Po(OH)Cl _x]^{3- x} while complex II was formulated as [PoCl_{2+ x}]^{2- x}. In this work, we study the ground-state geometries of the [Po(OH)Cl _x]^{3- x} and [PoCl_{2+ x}]^{2- x} complexes for x = 4-2, i.e. for the most probable complexes, with density functional theory (DFT), demonstrating that solvation can remarkably change the geometries of such systems. The electronic excitation energies are then computed with time-dependent DFT (TD-DFT), second-order N-electron valence state perturbation theory (NEVPT2), and contracted spin-orbit configuration interaction (c-SOCI), showing (i) that the SOC must be at play to obtain excitation energies in the right energy domain and (ii) that the quantum chemical calculations point toward x = 4, i.e., toward the [Po(OH)Cl₄]^- and [PoCl₆]^2- complexes.