Ultraviolet dechlorination of tetrachloro-p-benzoquinone by hydrogen sulfide: Theoretical confirmation of the significance of hydrosulfide radical

In this work, a systematical investigation on the role of hydrogen sulfide (H₂S) on the transformation of tetrachloro-p-benzoquinone (TCBQ) under ultraviolet (UV) irradiation (at 253.7 nm) in aqueous solution has been conducted through quantum chemical calculations. Under the UV irradiation, with the forward energy barrier (E_a,f, 11.7 kcal mol⁻¹) much lower than the reverse one (E_a,r, 22.3 kcal mol⁻¹), the first triplet state of TCBQ was kinetically feasible to react with bisulfide anion (HS⁻) via the Michael addition, and the addition of HS⁻ could promote the release of Cl⁻ and the formation of primary dechlorination product (HS-TriCBQ). During the UV photolysis of the primary dechlorination products (HO-TriCBQ and HS-TriCBQ) in the presence of H₂O and H₂S, the addition of nucleophile (OH⁻ or HS⁻) to the ortho-position of the hydroxyl or thiol group might be the most efficient pathway for the dechlorination, and their respective E_a,f were 9.2 kcal mol⁻¹ (for HS⁻-hydroxyl), 1.1 kcal mol⁻¹ (for OH⁻-thiol) and 8.9 kcal mol⁻¹ (for HS⁻-thiol). Moreover, the electron transfer from HS⁻ to the first triplet states could generate hydrosulfide radical for the dechlorination of TCBQ. The findings in the present study may provide some important theoretical foundation for the dehalogenation of TCBQ as well as other halobenzoquinones.