Site-Specific Carbon–Carbon Bond Fission in Photoexcited Propyl Radicals Leads to Isomer-Selective Carbene and Radical Products

Abstract

Although there have been many studies of C–H bond fission in the UV photochemistry of alkyl radicals, very little is known about the possible occurrence of C–C bond fission. Here, we report that upon excitation at 248 nm, gaseous 1-propyl radicals primarily undergo C–C bond fission, producing methylene (CH2) and ethyl radicals (C2H5), rather than the more energetically favored methyl (CH3) and ethylene (C2H4). In contrast, the exclusive C–C bond fission products from 2-propyl radicals were ethylidene (CHCH3) plus methyl radicals (CH3). The isomer-selective formation of high-energy carbene + radical products involves excited-state site-specific C–C bond fission at the radical carbon, with quantum yields comparable to those for C–H bond fission. Our observations suggest that a general feature of alkyl radical photochemistry is predissociation of the initially formed Rydberg states by high-lying valence states, yielding high-energy carbene plus alkyl radical products.