Oxygen dependence in the photoreaction of the pesticide metamitron

•Dependence of concentration, temperature and oxygen on photoreaction of metamitron.•Oxygen dependent quantum yields in aqueous solution.•Confirmation of a photoreaction from the triplet state.•Recommendation of metamitron as an actinometer.

The dependence of the quantum yield of metamitron on oxygen concentration in the range cO2 = (0.005-12.6)· 10−4 M in aqueous solution was investigated with a polychromatic Xe-light source in the wavelength region 270-376 nm. The quantum yield of Φ = 0.022 ± 0.002 mol mol−1 at T = (293 ± 3) K and cO2=2.8·10−4 M is independent from metamitron concentration in the range of cM = 8 ·10−8 − 1.8 · 10−4 M. A weak temperature dependence of the photolysis of metamitron at constant oxygen concentration was found with an activation energy of EA = (3 ± 1) kJ mol−1. The yield (ν = 0.91) of the main product desamino-metamitron does not depend on start concentration of metamitron, oxygen concentration or temperature (in the range of T = 281.5-327 K). Oxygen is necessary in the photolysis of metamitron. Quantum yields for the photoreaction of metamitron increase from practically zero at low oxygen concentrations (Φ = 0.002 mol mol−1 at cO2<3·10−6 M) to a maximum of Φmax = 0.042 mol mol−1 at cO2=5.1·10−5 M and declines with increasing oxygen concentration to a value of Φ = 0.006 mol mol−1 in oxygen saturated solutions. Two reaction sequences with a reaction from the triplet state T1 and from a recently proposed transient triplet state T1′ were used to explain the oxygen-dependent rate constants of metamitron. Both sequences explain the oxygen dependence equally well and it was not possible to distinguish between both mechanisms. Aqueous solutions of metamitron are recommended as a promising actinometric system, especially suitable for measurements in sunlight.

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