Comparative Thermochemistry and Kinetics of Bromine and Iodine Reactions with Atmospheric Mercury

Bromine and iodine radicals oxidize gaseous mercury, influencing its lifetime and deposition. Using CCSD(T) and CASPT2 calculations combined with variational transition-state theory, we compare the thermochemistry and kinetics of key reactions forming and destroying Hg-halide species. We investigate the atmospheric oxidation of mercury Hg(0) by Br and I to yield the corresponding Hg(I) halides and the subsequent oxidation reactions yielding Hg(II) compounds (or Hg(0)) via I, Br, BrO, ClO, IO, NO₂, and HO₂. The rate coefficient for ·HgI + I· → HgI₂ (4.2 × 10^-13 cm³ molecule^-1 s^-1) is about half that for the bromine analogue, and ·HgI + IO· → IHgOI is roughly seven times slower than ·HgBr + BrO· → BrHgOBr. The combined electronic-structure and kinetic analysis demonstrates that the employed methods reproduce periodic halogen trends within chemical accuracy, supporting the conclusion that bromine remains the dominant oxidant of atmospheric mercury under current conditions.