Kinetic Properties for the Complete Series Reactions of Chlorophenols with OH Radicals-Relevance for Dioxin Formation

The chlorophenoxy radical (CPR) is a key intermediate species in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). In municipal waste incinerators, the reactions of chlorophenols with OH radicals play the most central role in the formation of chlorophenoxy radicals. In this paper, molecular orbital theory calculations have been performed to investigate the formation of chlorophenoxy radicals from the complete series reactions of 19 chlorophenol congeners with OH radicals. The single-point energy calculation was carried out at the MPWB1K/6-311+G(3df,2p) level on the basis of the MPWB1K/6-31+G(d,p) optimized geometries. The kinetic modeling of the PCDD/PCDF (PCDD/F for short) formation demands the knowledge of the rate parameters for the formation of chlorophenoxy radicals from chlorophenols. So, the kinetic properties of the reactions of chlorophenols with OH radicals were deduced over a wide temperature range of 600-1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT). This study shows that the chlorine substitution at the ortho position in chlorophenol not only has a significant effect on the structures of chlorophenols, prereactive intermediates, the transition states, and chlorophenoxy radicals, but also plays a decisive role in determining the rate parameters.