Mechanism and Thermal Rate Constants for the Complete Series Reactions of Chlorophenols with H

Abstract

Reactions of chlorophenols with atomic H are important initial steps for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in incinerators. Detailed insight into the mechanism and kinetic properties of crucial elementary steps is a prerequisite for understanding the formation of PCDD/Fs. In this paper, the complete series reactions of 19 chlorophenol congeners with atomic H have been studied theoretically using the density functional theory (DFT) method and the direct dynamics method. The profiles of the potential energy surface were constructed at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+6(d,p) level. Modeling of the PCDD/Fs formation requires kinetic information aboutthe elemental reactions. The rate constants were deduced over a wide temperature range of 600-1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT). The rate-temperature formulas were fitted for the first time. This study shows that the substitution pattern of the phenol has a significant effect on the strength and reactivity of the O-H bonds in chlorophenols. Intramolecular hydrogen bonding plays a decisive role in determining the reactivity of the O-H bonds for ortho-substituted phenols.