Atmospheric oxidation of hexachlorobenzene: New global source of pentachlorophenol.

Hexachlorobenzene is highly persistent, bioaccumulative, toxic and globally distributed, a model persistent organic pollutant. The major atmospheric removal process for hexachlorobenzene is its oxidation by hydroxyl radicals. Unfortunately, there is no information on the reaction mechanism of this important atmospheric process and the respective degradation rates were measured in a narrow temperature range not of environmental relevance. Thus, the geometries and energies of all stationary points significant for the atmospheric oxidation of hexachlorobenzene are optimized using MP2/6-311G(d,p) method. Furthermore, the single point energies were calculated with G3 method on the optimized minima and transition-states. It was demonstrated for the first time that the addition of hydroxyl radicals to hexachlorobenzene proceeds indirectly, via a prereaction complex. In the prereaction complex the hydroxyl radical is almost perpendicular to the aromatic ring while oxygen is pointing to its center. In contrast, in the transition state it is nearly parallel with the aromatic ring. The reliable rate constants are calculated for the first time for the atmospheric oxidation of hexachlorobenzene for all environmentally relevant temperatures. It was also demonstrated for the first time that pentachlorophenol is the major stable product in the addition of hydroxyl radicals to hexachlorobenzene and that atmosphere seems to be a new global secondary source of pentachlorophenol.