Spectroscopic and kinetic characteristics of aroyloxyl radicals. 1. The 4-methoxybenzoyloxyl radical

A detailed analysis of the time-resolved, UV-visible spectrum obtained by 308-nm laser flash photolysis of bis-(4-methoxybenzoyl) peroxide proves that the broad, structureless absorption in the 500-800-nm region is due to the 4-methoxybenzoyloxyl radical. This radical also has an absorption at 320 nm. The long-wavelength absorption, for which there is less interference from other light-absorbing transients, has been used to measure absolute rate constants, k, for the reaction of 4-methoxybenzoyloxyl with a wide variety of organic substrates at ambient temperatures, e.g., cyclohexane, benzene, triethylsilane, cyclohexene, and 1,3-cyclohexadiene for which k in CCl 4 = (5.3 \ub1 3.0) 7 10 5, (2.3 \ub1 0.2) 7 10 6, (4.8 \ub1 0.1) 7 10 6, (6.4 \ub1 0.3) 7 10 7, and (4.8 \ub1 0.2) 7 10 8 M -1 s -1, respectively. Compared with the tert-butoxyl radical the 4-methoxybenzoyloxyl radical is about as reactive in hydrogen atom abstractions but is very much more reactive in additions to multiple bonds. The rate constant for decarboxylation of 4-methoxybenzoyloxyl at 24\ub0C is (3.4 \ub1 0.1) 7 10 5 s -1 in CCl 4 but is reduced to 642 7 10 4 s -1 in CH 3CN. The 4-methoxybenzoyloxyl radical can also be photodecarboxylated by using 700-nm light from a second laser or by using high power levels in the primary laser.Peer reviewed: YesNRC publication: Ye