Solvent-accelerated photoreduction of Hg( ii ) dihalides: uncovering solvent-governed and light-triggered mercury chemistry

Mercury dihalides (HgX₂, X = Cl, Br, I) undergo photoreduction much more rapidly in aqueous environments than in the gas phase. Using ab initio molecular dynamics simulations and high-level electronic structure calculations, we investigate how solvation shapes the molecular structure, electronic distribution, and excited-state character of HgX₂ complexes. We find that strong Hg-solvent interactions induce pronounced deviations from linear geometries and lead to partial negative charge accumulation on HgX₂ in polar solution. Moreover, we identify that the second absorption band in the deep-UV region exhibits a strong solvent-to-solute charge-transfer (CT) character. Combining the accumulation of partial negative charge in the ground state with the enhanced solvent-to-solute CT character promotes efficient electron localization on the Hg center after photoexcitation, thereby accelerating photoreduction in solution. By providing atomistic insight into solvation-driven excited-state reactivity, this work establishes the molecular basis for the accelerated photochemistry of HgX₂ in aqueous media and underscores the essential role of explicit solvation in modeling the solution-phase photochemistry of mercury species relevant to the global mercury cycle.