Kinetics of the dye sensitized photooxidation of 2-amino-4-hydroxy-6-methylpyrimidine, a model compound for some fungicides

The kinetics of the dye-sensitized photooxidation of 2-amino-4-hydroxy-6-me thyl-pyrimidine (AHMPD), a compound with the basic molecular structure of s ome systemic pyrimidine fungicides, has been studied in solution in water a nd in the mixture acetonitrile-water 4:1 v/v. Rate constants in the range 9 x 10(4)-2.7 x 10(7) M-1 s(-1) for both the overall and the reactive single t molecular oxygen [O-2((1)Delta(g))] quenching processes were determined b y time-resolved O-2((1)Delta(g))-phosphorescence detection and polarographi c methods. Photooxidation quantum yields were in the range 0.005-0.41, with values significantly higher in alkalinized media. On the contrary, pyrimid ine, 2-aminopyrimidine, 4-hydroxypyrimidin and 6-methylpyrimidine do not re act with O-2((1)Delta(g)), in neutral or alkalinized D2O solutions. The pre sence of a 4-hydroxy group in pyrimidines plays a key role in the photooxid ative process, because the formation of the tautomeric 4-oxo form, the pred ominant one in aqueous solution, diminishes or suppresses the interaction w ith O-2((1)Delta(g)) However, in the presence of alkali the OH-ionization g reatly enhances the photooxidation. The experimental evidence suggests a ch arge-transfer mediated mechanism involving an initial encounter excited com plex. These results also show that the sensitized photooxidation is an alte rnative pathway for the environmental or programmed degradation of these co lorless N-heteroaromatic compounds, in special in OH-ionized form. (C) 2000 Elsevier Science S.A. All rights reserved.