Experimental and theoretical investigation on photodegradation mechanisms of naproxen and its photoproducts

Abstract

The photochemical degradation of the pharmaceuticals and personal care products (PPCPs) has attracted increasing attention. In this study, a deep inspection of the photolysis mechanisms of naproxen and its photoproducts has been performed by employing experimental and theoretical methods. Contributions of different reactive oxygen species (ROS, such as OH, 1O2, and O2−) in the photolysis reaction also have been clarified. Based on the detected intermediates and DFT calculations, several photodegradation pathways of naproxen and its photoproducts have been proved. Furthermore, the deprotonated form of naproxen has been confirmed to be more reactive than the protonated one, and the lowest triplet state of naproxen is the reactive state. The decarboxylation mechanism of naproxen has been fully discussed. Meanwhile, the free energy barriers of OH-induced photolysis reactions (ΔG‡eff(1a) = 7.6 kcal mol−1, ΔG‡eff(4a) = 7.0 kcal mol−1) are much lower than the free energy barriers induced by O2− and 1O2. It proves that OH is the most favourable one among the three ROS. The similar inhabition rates and free energy barriers of reactions induced by O2− and 1O2, respectively, have demonstrated that O2− and 1O2 equally contribute to the degradation. Additionally, the computational results are coincident with the observed experimental findings. Hence, this work has verified a part of naproxen photodegradation mechanism under UV irradiation and brought about a rational way to investigate contributions of different ROS in the complex photochemical system of PPCPs.