Thermodynamics of reactions of ClHg and BrHg radicals with atmospherically abundant free radicals

Abstract. Quantum calculations are used to determine the stability of reactive gaseous mercury (Hg(II)) compounds likely to be formed in the Br-initiated oxidation of gaseous elemental mercury (Hg(0)). Due to the absence of any evidence, current models neglect the possible reaction of BrHg with abundant radicals such as NO, NO₂, HO₂, ClO, or BrO. The present work demonstrates that BrHg forms stable compounds, BrHgY, with all of these radicals except NO. Additional calculations on the analogous ClHgY compounds reveal that the strength of the XHg-Y bond (for X = Cl, Br) varies little with the identity of the halogen. Calculations further suggest that HO₂ and NO₃ do not form strong bonds with Hg(0), and cannot initiate Hg(0) oxidation in the gas phase. The theoretical approach is validated by comparison to published data on HgX₂ compounds, both from experiment and highly refined quantum chemical calculations. Quantum calculations on the stability of the anions of XHgY are carried out in order to aid future laboratory studies aimed at molecular-level characterization of gaseous Hg(II) compounds. Spectroscopic data on BrHg is analyzed to determine the equilibrium constant for its formation, and BrHg is determined to be much less stable than previously estimated. An expression is presented for the rate constant for BrHg dissociation.