Femtosecond Infrared Spectroscopy of Aroylperoxide Photofragmentation – Site Selective Decarboxylation

Abstract

Abstract The ultrafast photofragmentation of acetyl-benzoyl peroxide (ABPO) and 2-naphthoyl-benzoyl peroxide (NBPO) after UV excitation at 266 nm is studied by mid-infrared transient absorption spectroscopy. 13C-isotopic labelling is employed to unravel the primary fragmentation paths. For NBPO it appears that excitation of the benzoyloxy part causes decarboxylation exclusively at that site. In ABPO the acetyloxy part completely decarboxylates upon 266 nm excitation while only 80% – 85% of the benzoyloxyl radicals formed decarboxylate. The remaining radicals either combine with methyl radicals to methylbenzoate within about 30 ps or survive on the time scale of the experiment (1.5 ns). Photofragmentation of ABPO occurs from the S1-state whose lifetime is about 200 fs. All fragmentation products including CO2 also appear on this time scale.