Mechanism and direct kinetic study on the homogeneous gas-phase formation of 1,2,3,4,5,6,7-HpCN from the self-condensation of 2,3,4,5-TeCPRs and cross-condensation of PCPR with 2,3,4,5-TeCPR

Polychlorinated naphthalenes (PCNs) are ubiquitous in the environment and have received extensive public concern due to their dioxin-like properties. 1,2,3,4,5,6,7-heptachloronaphthalene (1,2,3,4,5,6,7-HpCN) is one of the most toxic and detected PCNs. The chlorophenoxy radical (CPR) was demonstrated as a crucial intermediate specie for PCN formation in thermal and combustion conditions. In this study, the homogeneous gas-phase formations of PCNs from the self-condensation of 2,3,4,5-terachlorophenoxy radicals (2,3,4,5-TeCPRs) and the cross-condensation of pentachlorophenoxy radical (PCPR) with 2,3,4,5-TeCPR were researched by using the direct density functional theory (DFT) kinetic calculation. The possible formation mechanisms were investigated at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants and their temperature dependence of key elementary reactions were deduced over the temperature range of 600-1200 K. This study reveals that pathways concluding with Cl elimination are energetically favored over those ending with H elimination. 1,2,3,4,5,6,7-HpCN is the main PCN product from the self-condensation of 2,3,4,5-TeCPRs, whereas 1,2,3,4,5,6,7-HpCN and 1,2,3,4,5,6,8-HpCN are the major PCN products from the cross-condensation of PCPR and 2,3,4,5-TeCPR. 1,2,3,4,5,6,7-HpCN is preferentially formed from self-condensation of 2,3,4,5-TeCPRs. Multi- chlorine substitutions suppress the PCN formations. The findings discussed in this paper aim to provide detailed insights into the formation mechanism of highly chlorinated PCNs from chlorophenol precursors under thermal and combustion conditions.