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-methyl-pyrimidine (AHMPD), a compound with the basic molecular structure of some systemic pyrimidine fungicides, has been studied in solution in water and 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 singlet molecular oxygen [O-2((1)Delta(g))] quenching processes were determined by time-resolved O-2((1)Delta(g))-phosphorescence detection and polarographic methods. Photooxidation quantum yields were in the range 0.005-0.41, with values significantly higher in alkalinized media. On the contrary, pyrimidine, 2-aminopyrimidine, 4-hydroxypyrimidin and 6-methylpyrimidine do not react with O-2((1)Delta(g)), in neutral or alkalinized D2O solutions. The presence of a 4-hydroxy group in pyrimidines plays a key role in the photooxidative process, because the formation of the tautomeric 4-oxo form, the predominant one in aqueous solution, diminishes or suppresses the interaction with O-2((1)Delta(g)) However, in the presence of alkali the OH-ionization greatly enhances the photooxidation. The experimental evidence suggests a charge-transfer mediated mechanism involving an initial encounter excited complex. These results also show that the sensitized photooxidation is an alternative pathway for the environmental or programmed degradation of these colorless N-heteroaromatic compounds, in special in OH-ionized form. (C) 2000 Elsevier Science S.A. All rights reserved.