Tropospheric Degradation of Perfluorinated Aromatics: A Case of Hexafluorobenzene

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.