A kinetic study of the photolysis of sulfamethoxazole with special emphasis on the photoisomer.

The previously not studied photochemical degradation of sulfamethoxazole (SMX) to the isomer of SMX (ISO) was measured via a polychromatic (Xe) and a monochromatic (Hg) light source and accompanied by quantum chemical DFT calculations. In addition to the [Formula: see text] of ISO, tautomer-dependent properties such as the [Formula: see text] were measured and theoretically confirmed by DFT. The kinetics in solutions below and above the [Formula: see text] of SMX were studied for the available and quantifiable products SMX, ISO, 3-amino-5-methylisoxazole (AMI), 2-amino-5-methyloxazole (AMO), and sulfanilic acid (SUA). The quantum yields of the neutral ([Formula: see text]) and anionic [Formula: see text]) forms of SMX ([Formula: see text], [Formula: see text]) and ISO ([Formula: see text] and [Formula: see text]) were found to be wavelength-independent. In a competitive reaction to the formation of ISO from SMX, the degradation product TP271 is formed. Various proposed structures for TP271 described in the literature have been studied quantum mechanically and can be excluded for thermodynamic reasons. In real samples in a northern German surface water in summer 2021 mean concentrations of SMX were found in the range of 120 ng/L. In agreement with the pH-dependent yields, concentrations of ISO were low in the range of 8 ng/L.