Computational Study on the Photolysis of BrHgONO and the Reactions of BrHgO^• with CH₄, C₂H₆, NO, and NO₂: Implications for Formation of Hg(II) Compounds in the Atmosphere

Abstract

Global models suggest BrHgONO to be the major Hg­(II) species initially formed in atmospheric oxidation of Hg(0) in most of the atmosphere, but its atmospheric fate has not been previously investigated. In the present work, we use quantum chemistry to predict that BrHgONO photolysis produces the thermally stable radical BrHgO•. Subsequently, BrHgO• may react with NO2 to form thermally stable BrHgONO2, or with NO to re-form BrHgONO. Additionally, BrHgO• abstracts hydrogen atoms from CH4 and C2H6 with higher rate constants than does •OH, producing a stable BrHgOH molecule. Because BrHgO• can abstract hydrogen atoms from sp3-hybridized carbons on many organic compounds, we expect production of BrHgOH to dominate globally, although formation of BrHgONO and BrHgONO2 may compete in urban regions. In the absence of experimental data on the kinetics and fate of BrHgONO and BrHgO•, we aim to guide modelers and other scientists in their search for Hg­(II) compounds in the atmosphere.

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury

“…For syn ‐HgBrONO, two types of transitions of distinct nature result in the population of S 1 ; i) a mainly monoconfigurational transition in which the electron excitation is localized in the ONO part of the molecule, the electronic density redistribution takes place from an in‐plane oxygen lone pair orbital of the N=O moiety to the antibonding π* orbital (see the violet box in Figure a), and ii) a multiconfigurational transition described mainly by the excitation of type (i) plus a second excitation in which now the electron departs from an orbital with contributions also from the Hg and Br atoms (delocalized orbital) to the same π* orbital (see the pink box in Figure a). While type (i) occurs at the Franck–Condon geometry (band maximum), type (ii) appears in other surrounding geometries and contributes to the increase in the intensity of the S 1 band. Type (ii) contributions, from excitations of the HgBr part of the molecule, shall affect the photodynamics of the system, as shown below.…”

“…The remaining trajectories (10 %) yield HgBr upon release of NO 2 . These results confirm previous CCSD(T) estimations, which pointed to HgBrO as the main photoproduct . However, the present dynamic calculations allow us to quantify the specific yield of each reaction channel.…”

“…Only for syn ‐HgBrONO, recent computational work indicated that dissociation might occur rapidly in the lower atmosphere giving rise to HgBrO. This radical would further react with atmospheric trace gases, such as NO, NO 2 , and volatile organic compounds, to finally yield HgBrOH . However, in order to obtain a comprehensive and quantitative description of the atmospheric Hg chemical mechanism, the spectral properties and photodissociation reaction products of syn ‐HgBrONO, HgBrOOH, HgBrO, and HgBrOH need to be known, which are currently unexplored.…”

“…The HgBrO radical appears as an important product in the photolysis of syn ‐HgBrONO and HgBrOOH, and it has been suggested to further react with NO, NO 2 , and volatile organic compounds within the lower troposphere to yield HgBrONO, HgBrONO 2 , and HgBrOH, respectively . On the other hand, the UV/Vis absorption spectrum and photochemical properties of this radical remained unknown.…”

“…Previous computational studies have focused mainly on the thermal reactivity of closed‐shell molecules . Only for syn ‐HgBrONO, recent computational work indicated that dissociation might occur rapidly in the lower atmosphere giving rise to HgBrO. This radical would further react with atmospheric trace gases, such as NO, NO 2 , and volatile organic compounds, to finally yield HgBrOH .…”