Aqueous Degradation of Flumequine Induced by Hydroxyl and Sulfate Radicals: A Theoretical Research

Abstract

Density functional theory (DFT) calculation at the M06-2X/6-311+G(d,p) level was carried out to investigate aqueous degradation reactions of neutral and anionic forms of flumequine (FLU(N) and FLU(A)) induced by hydroxyl (\(^{ \bullet }{\kern 1pt} {\text{OH}}\)) and sulfate radicals (\({\text{SO}}_{4}^{{ \bullet - }}\)), including \(^{ \bullet }{\kern 1pt} {\text{OH}}\) (\({\text{SO}}_{4}^{{ \bullet - }}\)) addition (ADD), H atom abstraction (HAA) and single electron transfer (SET). The oxidation performance of \(^{ \bullet }{\kern 1pt} {\text{OH}}\) and \({\text{SO}}_{4}^{{ \bullet - }}\) as an oxidant was analyzed. The computational results indicated that the degradation reactions of FLU(N) and FLU(A) involving \(^{ \bullet }{\kern 1pt} {\text{OH}}\) (\({\text{SO}}_{4}^{{ \bullet - }}\)) have some common features, namely, two ADD pathways involving C2 and C4 positions or C1 and C2 atoms in all six addition reactions are kinetically the most favorable, while the ADD pathway related to C3 atom is always the most unfavorable. Two HAA pathways associated with C7 and C9 atoms are the most advantageous in all HAA reactions. Moreover, the ADD pathway involving a C(sp²)−H bond is always easier to proceed than the HAA channel related to the same C atom. In addition, all SET reactions between FLU(N) or FLU (A) and \(^{ \bullet }{\kern 1pt} {\text{OH}}\) (\({\text{SO}}_{4}^{{ \bullet - }}\)) were found to have the highest Gibbs free energy of activation (Δ_rG^≠) compared with other parallel channels, and hence could be ruled out from the degradation mechanism of FLU. In most cases, the ADD pathway is the most favorable of all possible reaction channels. The reactivity of FLU(A) was found to be higher than that of FLU(N). Compared with \({\text{SO}}_{4}^{{ \bullet - }}\), \(^{ \bullet }{\kern 1pt} {\text{OH}}\) is a stronger oxidant in degrading FLU(N). However, the opposite situation can be observed for degradation of FLU(A). Hydroxylation and ring-opening of quinolone ring are important subsequent processes for favorable ADD and HAA intermediates.