Real-Time Probing of the Multistep/Multichannel Photoisomerization and Dissociation of Propenal

The photoinitiated unimolecular decomposition of propenal, the simplest unsaturated organic carbonyl, is of potential significance in our atmosphere and in the interstellar medium. Synergistic experimental and theoretical studies reveal complex multistep dynamics involving isomerization to methylketene and subsequent elimination of CO to form ethylidene, CH3CH, the elusive high energy carbene isomer of ethylene, CH2CH2. Energy-resolved real-time monitoring of methylketene reaction intermediates yields decay constants that are in excellent accordance with state-of-the-art first-principles RRKM calculations for a sequence of reaction steps occurring over relatively long timescales. Isomerization primarily involves two sequential α-H atom migrations via methylformylcarbene, rather than from direct 3-center H atom transfer. Due to near coincidence of the barrier height for formation of triplet ethylidene and the endoergicity for formation of electronically excited singlet ethylidene, both channels compete over a wide range of energies, corresponding to excitation wavelengths between 340 and 370 nm.