TY - JOUR

T1 - Oxygen dependence in the photoreaction of the pesticide metamitron

AU - Palm, Wolf-Ulrich

PY - 2017/10/1

Y1 - 2017/10/1

N2 - 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 −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.

AB - 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 −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.

KW - Sustainability Science

KW - Actinometer

KW - Desamino-metamitron

KW - Metamitron

KW - Oxygen

KW - Chemistry

UR - http://www.scopus.com/inward/record.url?scp=85026764101&partnerID=8YFLogxK

U2 - 10.1016/j.jphotochem.2017.07.033

DO - 10.1016/j.jphotochem.2017.07.033

M3 - Journal articles

AN - SCOPUS:85026764101

VL - 347

SP - 138

EP - 145

JO - Journal of Photochemistry and Photobiology A: Chemistry

JF - Journal of Photochemistry and Photobiology A: Chemistry

SN - 1010-6030

ER -