Bromine atom initiates atmospheric oxidation of Hg(0) in mercury depletion events

Quantum chemical calculations have been carried out to investigate the thermochemistry of BrHg• forming bonds with atmospherically abundant radicals Y• (Y = NO2, HO2, ClO, BrO, or IO). The coupled cluster method with single and double excitations (CCSD), combining with relativistic effective core potentials, is used to determine the equilibrium geometries and harmonic vibrational frequencies of BrHgY species. The BrHg-Y bond enthalpies are refined using CCSD with a non-iterative estimate of the triple excitations (CCSD(T)) combined with correlation consistent basis sets that extrapolate to the complete basis set limit. Spin-orbit coupling is accounted for by comparing results of two-component spin-orbit density functional theory (DFT) calculations with results from standard DFT. We also assess the performances of various DFT methods for calculating molecular structures and vibrational frequencies of BrHgY species. The results of present work will provide a reference for bromine-initiated atmospheric chemistry of mercury.