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.
Im et al. (2026) studied this question.