Physical Chemistry Chemical Physics 2018-04-13

Phenyl Radical + Propene: A Prototypical Reaction Surface for Aromatic‐Catalyzed 1,2‐Hydrogen‐Migration and Subsequent Resonance‐Stabilized Radical Formation

Zachary J. Buras, Te-Chun Chu, Adeel Jamal, Nathan W. Yee, Joshua E. Middaugh, William H. Green

Index: 10.1039/C8CP01159A

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Abstract

The C9H11 potential energy surface (PES) was experimentally and theoretically explored because it is a relatively simple, prototypical alkylaromatic radical system. Although the C9H11 PES has already been extensively studied both experimentally (under single‐collision and thermal conditions) and theoretically, new insights were made in this work by taking a new experimental approach: flash photolysis combined with time‐resolved molecular beam mass spectrometry (MBMS) and visible laser absorbance. The C9H11 PES was experimentally accessed by photolytic generation of phenyl radical and subsequent reaction with excess propene (C6H5 + C3H6). The overall kinetics of C6H5 + C3H6 was measured using laser absorbance with high time‐resolution from 300‐700 K and found to be in agreement with earlier measurements over a lower temperature range. Five major product channels of C6H5 + C3H6 were observed with MBMS at 600 and 700 K, four of which were expected: Hydrogen (H) ‐ abstraction (measured by the stable benzene, C6H6, product), methyl radical (CH3) ‐ loss (styrene detected), H‐loss (phenylpropene isomers detected) and radical adduct stabilization. The fifth, unexpected prouct observed was benzyl radical, which was rationalized by the inclusion of a previously‐unreported pathway on the C9H11 PES: aromatic‐catalyed 1,2‐H‐migraton and subsequent resonance stabilized radical (RSR, benzyl radical in this case) formation. The current theoretical understanding of the C9H11 PES was supported (including the aromatic‐catalyzed pathway) by quantitative comparisons between modelled and experimental MBMS results. At 700 K, the branching to styrene + CH3 was 2‐4 times greater than that of any other product channel, while benzyl radical + C2H4 from the aromaticcatalyzed pathway accounted for ~10% of the branching. Single-collision conditions were also simulated on the updated PES to explain why previous crossed molecular beam experiments did not see evidence of the aromatic-catalyzed pathway. This experimentally‐validated knowledge of the C9H11 PES was added to the database of the open‐source Reaction Mechanism Generator (RMG), which was then used to generalize the findings on the C9H11 PES to a slightly more complicated alkylaromatic system.