Abstract
Chlorothiophenols (CTPs) are known to be key and direct precursors of polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs). Self/cross-coupling of the chlorothiophenoxy radicals (CTPRs), sulfydryl-substituted phenyl radicals and thiophenoxyl diradicals evolving from CTPs are initial and important steps for PCTA/DT formation. In this study, quantum chemical calculations were carried out to investigate the homogenous gas-phase formation of PCTA/DTs from self/cross-coupling of 2,4-dichlorothiophenoxy radical (R1), 2-sulfydryl-3,5-dichlorophenyl radical (R2) and 3,5-dichlorothiophenoxyl diradical (DR) at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of crucial elementary steps were deduced over 600–1200 K, using canonical variational transition state theory with a small curvature tunneling contribution. For the formation of PCTAs, the S•/σ-C• condensation with both thiophenolic sulfur in one radical and ortho carbon in the other radical bonded to single electron is the most efficient sulfur-carbon coupling mode, and the ranking of the PCTA formation potential is DR + DR > R2 + DR > R1 + DR > R1 + R2 > R1 + R1. For the formation of PCDTs, the σ-C•/σ-C• coupling with both ortho carbon in the two radicals bonded to single electron is the energetically favored carbon-carbon coupling mode, and the ranking of the PCDT formation potential is: R2 + DR > R2 + R2 > R1 + DR > R1 + R2 > R1 + R1. The PCTA/DTs could be produced from R1, R2 and DR much more readily than PCDD/DFs from corresponding oxygen substituted radicals.
Highlights
Polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs) are two groups of sulfur-substituted structural analogues of the polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/DFs), which are persistent, lipophilic and considered to be dioxin-like compounds [1]
We present a systematic theoretical study on the PCTA/DT formation mechanism from 2,4-dichlorothiophenol (2,4-DCTP) as precursor by the self-/cross- condensation of 2,4-dichlorothiophenoxy radical (R1), 2-sulfydryl-3,5-dichlorophenyl radical (R2) and 3,5-dichlorothiophenoxyl diradical (DR). 2,4-DCTP was selected as the model because it has the minimum number of Cl atoms to form 2,4,6,8-TeCDT, which was the most widely detected and important PCTA/DTs in the environment
The reliability and accuracy of the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) level for the geometries, frequencies and energy calculation in this study have been confirmed in our previous works on PCTA/DT formation with 2,4-DCTP as the precursor [29,39]
Summary
Polychlorinated thianthrene/dibenzothiophenes (PCTA/DTs) are two groups of sulfur-substituted structural analogues of the polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/DFs), which are persistent, lipophilic and considered to be dioxin-like compounds [1]. PCTA/DTs exhibit less toxicity than PCDD/DFs with the toxicity equivalent factor (TEF) of 2,3,7,8-TeCTA and 2,3,7,8-TeCDT as 0.01 and 0.001, respectively [2]. PCTA/DTs have similar and even much higher concentration in some regions and bioaccumulation potential than PCDD/DFs [4,13], which have been detected in the different environmental samples, including soil and sediment [11,14], pulp bleaching [1], incineration of municipal waste [15,16], wastes from petroleum refineries [17] and petroleum spills [18]. Pruell et al observed that the mean concentrations of 2,4,6,8-TeCDT (3680 ng/kg) in Passaic River sediments were 5-10 times higher than that of 2,3,7,8-TeCDD (656 ng/kg) and 2,3,7,8-TeCDF (334 ng/kg) [11]. Considering the serious environmental pollution and wide concern by scientific researchers of these toxic substances, it is significant to clarify the formation mechanism of PCTA/DTs under combustion and thermal processes to hinder their harm to humans and the environment
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