BrHgO^• + CO: Analogue of OH + CO and Reduction Path for Hg(II) in the Atmosphere

We present results of the first study of the reaction 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. Much of the potential energy surface mimics that for the ubiquitous OH + CO → H + CO₂ reaction, including the entrance channel and binding energies of intermediates. However, the BrHgOCO intermediate is much less stable than HOCO with respect to loss of CO₂. This leads to ultrafast dissociation of BrHgOCO and prevents its stabilization in air (unlike HOCO). Because of the relatively high rate constant for BrHgO^• + CO and the high abundance of CO throughout the troposphere, this reaction could dominate the atmospheric fate of BrHgO^•. The BrHg^• product of this reaction can dissociate to form Hg(0), and Hg(0) is transferred to ecosystems much more slowly than Hg­(II) compounds. Therefore, this reaction could significantly slow the transfer of neurotoxic mercury from the atmosphere to ecosystems.