First stages of photodegradation of the urea herbicides Fenuron, Monuron and Diuron

The initial stages of photodegradation of three urea-based herbicides [ArNHCON(CH3)(2) - (1)]: Fenuron, Monuron and Diuron have been studied using laser-flash photolysis and pulse radiolysis. Radical cations 2 are generated immediately after the laser pulse, and deprotonate to yield neutral radicals, 3. The quantum yield of 193-nm photoionization of 1 to yield 2 (phi (PI)) are ca. 10%. Pulse-radiolytic one-e - oxidation with SOI also yields 2 and, after deprotonation, 3. Semiempirical simulations of UV/Vis spectra of 2 and 3 support the experimental observations. A bathochromic displacement of the bands of 2 is observed as the number of e(-) -withdrawing ring substituents increases. The extinction coefficients of 2 and 3 are, respectively, ca. 3000 and ca. 2000 M-1 cm(-1). The pK(a) for deprotonation of 3 is ca. 0. (C) 2001 Elsevier Science B.V. All rights reserved.