Theoretical Study On The Reaction Mechanism Of TCDD/F And CH Radicals

With the fast development of the global economy,the environmental pollution has become more prominent increasingly,where the harms induced by the persistent organic pollutants(POPs)are the most serious.Taking the typical dioxin pollutants in POPs for examples,i.e.,2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD)and 2,3,7,8-tetrachlorodibenzofuran(TCDF)have been studied extensively due to their high toxicity,migration,bioaccumulation,other adverse biological effects,and widespread occurrence in the environment.Therefore,the control,reduction,and conversion of the these dioxins have become one of the important global environmental problems.Owing to the rather low vapor pressure and high toxicity of the dioxins,the relevant reactions are difficult to explore experimentally.In this case,it is necessary to carry out the relvent studies employing the quantum chemistry calculations.To look for a better method to degradate the dioxins POPs,in this study,the reactions of the TCDD and TCDF with the CH radical have been systematically investigated theoretically for the first time based on the unique stability of the dioxins and the high reactivity of CH radicals.The density functional theory(DFT)B3LYP method and the 6-311++G** basis set in combination with the atoms in molecules(AIM)theory,ab initio molecular dynamics,and reduced density gradient(RDG)analyses have been employed.The detailed reaction mechanisms,geometrical characteristics,energy properties,IR spectra,and the hyperfine coupling constants for the dominant products have been discussed,respectively.The present studies mainly include the following two parts:In the first part,the reaction mechanisms of TCDD with CH radical have been systematically investigated.It was found that it is a multi-channel reaction,i.e.,the CH radical can attack the C-X(X=C,Cl,H,O)bonds of TCDD via the insertion modes,resulting in the formation of 8 products.Thermodynamically,the whole reaction processes are exothermic and spontaneous since all the enthalpy and Gibbs free energy changes are negative values in the formation processes.Furthermore,the thermodynamic stability of the products is controlled by the distribution of the single unpaired electron.Kinetically,the most favorable reaction channel is the insertion of the CH radical into the C-C bond,resulting in the formation of three seven-membered ring compounds.Moreover,the reactivity of the CH radical toward the F-and Br-substituted TCDDs has also been investigated.In addition,the dominant products of the reaction have been confirmed by the ab initio molecular dynamics simulation.The calculated IR spectra and hyperfine coupling constants of the dominant products can provide convenience for the detection and identification of the products experimentally.In the second part,the reaction mechanisms of TCDF with CH radical have been systematically investigaged.Similar to the reaction of TCDD with CH radical,this reaction is also a multi-channel reaction,i.e.,the CH radical can attack the C-X(X=C,Cl,H,O)bonds of TCDF via the insertion modes,resulting in the formation of 13 products.Thermodynamically,the whole reaction processes are exothermic and spontaneous since all the enthalpy and Gibbs free energy changes are negative values in the formation processes.Furthermore,the thermodynamic stability of the products is controlled by the distribution of the single unpaired electron.Kinetically,the most favorable reaction channel is the insertion of the CH radical into the C-C bond except for those directly attached to the chlorine atom,resulting in the formation of three seven-membered ring compounds.Moreover,the reactivity of the CH radical toward the Fand Br-substituted TCDFs has also been investigated.In addition,the dominant products of the reaction have been confirmed by the ab initio molecular dynamics simulation.The predicated IR spectra and hyperfine coupling constants of the dominant products can provide convenience for the detection and identification of the products experimentally.Expectedly,the present findings not only can help to better understand the transformation of the TCDD/F dioxins and the potential reactivity of the CH radical in the atmosphere,but also can provide theoretical proof and scientific guidance for the performance of the relevant experiments.