Quantum Chemistry to Support Photoionization Mass Spectrometry Studies of Atmospheric Mercury Compounds

We use computational quantum chemistry to determine the ionization potentials (IPs) of eight mercury-containing species that are thought to be formed in the atmospheric oxidation of mercury. These compounds are XHgO•, XHgOH, XHgONO, and XHgOOH (X = Br, OH), for which IPs vary from 9.1 to 10.8 eV, with little difference between X = Br and X = OH. We also determine the stability of the corresponding ions with respect to nearly 100 dissociation pathways. XHgO⁺ are the least stable ions, thermodynamically, requiring 0.5 eV (X = Br) or 0.8 (X = OH) eV to decompose, while XHgOH⁺ needs 1.0 eV (Hg(OH)₂) or 1.1 eV (BrHgOH) to decompose. The lowest energy dissociation paths for XHgO⁺ and XHgOH⁺ lead to Hg⁺ and a neutral fragment, so they all involve some rearrangement rather than being simple bond fission reactions. Laboratory studies have yet to detect any of the eight neutral compounds in the gas phase. Regional and global models of atmospheric mercury include production and destruction of these eight species, but rate constants for these reactions are mostly only known from computational chemistry or arguments by analogy. We hope the present work facilitates the design and interpretation of mass spectrometric studies of these compounds that will help, not only to obtain reliable rate constants for the reactions producing and destroying these eight compounds, but also, more fundamentally, to validate the mechanisms used in these models.