Tropospheric Degradation of Perfluorinated Aromatics: A Case of Hexafluorobenzene

Abstract

Abstract:The major tropospheric removal process for hexafluorobenzene is its oxidation by hydroxyl (OH) radicals. However, there is no information on the reaction mechanism of this important process. All geometries and energies significant for the tropospheric degradation of hexafluorobenzene were characterized using the MP2/6-311+G(d,p) and/or G3 methods. It was found out that the addition of OH radical to hexafluorobenzene proceeds via a prereaction complex. In the prereaction complex the OH radical is almost perpendicular to the aromatic ring and oxygen is pointing to its center. The reaction rate constants for addition of OH radical to hexafluorobenzene were determined for the temperature range 230-330 K, using RRKM theory and corrected G3 energies. For the whole range of environmentally relevant temperatures (230-330 K) there is a very good qualitative agreement between the calculated and experimental rate constants. Finally, our results almost perfectly reproduce the unusually weak temperature dependence for OH radical addition to hexafluorobenzene.

Atmospheric oxidation of halogenated aromatics: comparative analysis of reaction mechanisms and reaction kinetics

“…In monohalogenated benzenes, uorobenzene and chlorobenzene, the hydrogen atom is pointing approximately to the center of the aromatic ring 79,81 while for perhalogenated benzenes, hexauorobenzene and hexachlorobenzene, an oxygen atom is pointing approximately to the center of the aromatic ring. 83,84 The separations between hydroxyl radical hydrogen and the center of the aromatic ring are 2.35 and 2.44Å, respectively, for uorobenzene and chlorobenzene while the separations (1) between hydroxyl radical oxygen and the aromatic ring are somewhat larger for hexauorobenzene and hexachlorobenzene, i.e. 2.76 and 2.89Å, respectively.…”

“…Furthermore, the relative translation of the hydroxyl radical with respect to halogenated benzenes in the prereaction complexes was treated as a two-dimensional particle-in-the-box potential. 79,81,83,84 Reaction between the hydroxyl radical and individual halogenated benzene is divided into two steps (eqn (1)). The rst step is the association of reactants and formation of a prereaction complex and the corresponding rate constants were calculated by combining the statistical thermodynamics calculations using the Thermo program from the MULTIWELL program package [92][93][94] and RRKM method 95,96 in the high pressure limiting regime.…”

“…A more detailed description of computational methods and procedures can be found in the original studies. 79,81,83,84…”

“…3). 97 The formation of prereaction complexes proceeds across loose transition states 79,81,83,84,95,96 which are favorable for the formation of prereaction complexes since those transition states reduce the dissociation rate of prereaction complexes (eqn (1)). The loose transition states seem to be analogous to the outer transition state suggested for the addition of the hydroxyl radical to ethylene.…”

“…Those results coupled with the RRKM (Rice-Ramsperger-Kassel-Marcus) kinetic theory have successfully reproduced the measured reaction rates and unusual temperature dependence for the oxidation of uorobenzene 79 and chlorobenzene. 81 Recently, the same approach was also successfully applied to obtain valuable information on the reaction mechanism and reaction-path dynamics for atmospheric oxidation of perhalogenated benzenes, 83,84 one of them being hexachlorobenzene, a model persistent organic pollutant (POP) from the original list of the twelve major POPs listed under the Stockholm Convention. 85,86 In this study, a comparative analysis will be made on the reaction mechanisms, thermodynamic data, calculated reaction rates and their temperature dependence as well as on the potential stable products of the atmospheric oxidation of halogenated benzenes by OH radicals.…”