Quantum chemical calculations have been performed to investigate the photolysis mechanism of relatively susceptible sulfonamide moiety of five-membered sulfonamide (SA) antibiotics, such as sulfamethoxazole, sulfisoxazole, sulfamethizole, and sulfathiazole. The results show that the ·OH-mediated indirect photolysis of sulfonamide linkage has two possible multi-step reaction pathways, viz., H-abstraction and electrophilic C1-attack, which is contrast to previously reported one-step cleavage manner. The newly proposed indirect photolysis mechanisms could be applied to six-membered SAs such as sulfadimethoxine. It has been found that the dissociation of S

N bond is easier in direct photolysis than ·OH-mediated indirect photolysis. Wiberg bond index and LUMO-HOMO energy gap are investigated to clarify the origin of the discrepant reactivity of sulfonamide moiety of SAs at singlet and triplet states. In comparison with singlet states, the SN bond of SAs is weaker at triplet states of SAs and thus results in higher reactivity of sulfonamide moiety, as also suggested by smaller LUMO-HOMO energy gap. This study could add better understanding to the photolysis mechanisms of SAs, which would be also helpful in utilizing quantum chemistry calculation to investigate the behavior and fate of antibiotics in the aquatic environment.

中文翻译：

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五元磺酰胺中磺酰胺部分的光解机理：DFT研究

已经进行了量子化学计算，以研究五元磺酰胺（SA）抗生素（如磺胺甲恶唑，磺胺异恶唑，磺胺甲唑和磺胺噻唑）相对易感的磺酰胺部分的光解机理。结果表明，·OH介导的磺酰胺键间接光解具有两种可能的多步反应途径，即H吸收和亲电C1攻击，这与以前报道的一步裂解方式形成对比。新提出的间接光解机理可应用于六元SA，如磺胺二甲氧嘧啶。已经发现，S的解离

N键在直接光解中比·OH介导的间接光解更容易。研究了Wiberg键指数和LUMO-HOMO能隙，以阐明SA在单重态和三重态下磺酰胺部分的不同反应性的起源。与单重态相比，SSAs的N键在SA的三重态较弱，因此导致磺酰胺部分的反应性更高，这也由较小的LUMO-HOMO能隙暗示。这项研究可以加深对SAs的光解机理的了解，这也将有助于利用量子化学计算来研究抗生素在水生环境中的行为和命运。