High selective gas-phase rearrangement reaction of TCDD induced by excess electron attachment: Theoretical insight on the decomposition mechanism of one of the most toxic chemical known to science.

Abstract

Dioxins are highly toxic chemicals with serious health risks, for which there is no safe level of exposure. Because of the slow decomposition of dioxins, the removal of these persistent environmental pollutants still remains a challenge. Based on theoretical studies, the present work investigates the degradation mechanism of the most toxic type of dioxin-related compounds by low-energy electron irradiation. To explore the rearrangement manner of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced by excess electron attachment, the B3LYP-D3(BJ)/def2-TZVP//B3LYP-D3(BJ)/6-311++G(d,p) level of density functional theory was applied. Electron attachment resulted in a remarkable decrease in the activation barrier of the rearrangement reaction in a thermodynamically preferred reaction. An activation energy as low as 6.6\xa0kcal/mol provides a strong demonstration that this pathway is the most effective in comparison to the neutral or radical rearrangement mechanisms. The attachment of electrons in the above energy range to C-Cl σ∗orbital is more likely than attachment to the LUMO of TCDD with π∗ orbital character. This σ∗ anion then undergoes a C-O σ bond rupture over a small barrier to produce a relatively stable intermediate, n-IM. The formed n-IM is again reactive toward a subsequent C-O bond rupture along with an intramolecular C-C coupling to produce the products, (E)-4,4 ,5,5 -tetrachloro-[1,1 -bi(cyclohexylidene)]-3,3 ,5,5 -tetraene-2,2 -dione and (Z)-4,4 ,5,5 -tetrachloro-[1,1 -bi(cyclohexylidene)]-3,3 ,5,5 -tetraene-2,2 -dione. The thermodynamic driving force for the anionic mechanism ensures the formation of the products to be irreversible enough to be purified. The purified products, with their active carbonyl groups, can react in many different ways with a wide range of nucleophile compounds.