Atmospheric chemistry of oxygenated mercury-containing compounds

Mercury is transported globally through the atmosphere as atomic mercury, but mostly it is transferred from the atmosphere to ecosystems in the form of Hg(II) compounds. As a result, scientists are increasingly focused on oxidation-reduction chemistry of mercury in the atmosphere. At present, little is known about the interaction of mercury compounds with environmental surfaces, which commonly possess adsorbed water.

As a first step towards understanding these interactions, we have theoretically studied the reaction of BrHgO• + CO → BrHg• + CO₂, which constitutes a potentially important mercury reduction reaction in the atmosphere. We characterized the potential energy surface with CCSD(T)/CBS energies (with corrections for relativistic effects) at MP2 geometries. Master Equation simulations were used to reveal the factors controlling the overall rate constant.

In a second step and for the first time, the monohydration of several oxygenated mercury-containing compounds (BrHgO, BrHgOH, BrHgOOH, BrHgNO₂ and its isomers, and HgOH) with one water molecule has been theoretically studied using the ωB97X-D/aug‐cc‐pVTZ level of theory. The thermodynamic properties of the hydration reactions have been calculated using DFT geometries with energies with coupled-cluster calculations DK-CCSD(T) and the ANO‐RCC‐Large basis sets. Standard reaction enthalpy and standard Gibbs free reaction energy were computed. The temperature dependences of Δ_rG°(T) were evaluated for all studied aggregates over the temperature range 200 - 400 K. For the first time, the monohydration processes have been studied to elucidate the role of hydrating water molecules. Atmospheric implications have been discussed.