Oxygen dependence in the photoreaction of the pesticide metamitron

Abstract

Abstract The dependence of the quantum yield of metamitron on oxygen concentration in the range c O 2 = (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 c O 2 = 2.8 · 10 − 4 M is independent from metamitron concentration in the range of c M = 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 E A = (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 c O 2 3 · 10 − 6 M) to a maximum of Φ max = 0.042 mol mol −1 at c O 2 = 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 T 1 and from a recently proposed transient triplet state T 1 ′ 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.