Product Branching Fractions of the CH + Propene Reaction from Synchrotron Photoionization Mass Spectrometry

Abstract

The CH­(X²Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C₄H₆ (<i>m</i>/<i>z</i> 54) + H. By fitting experimental photoionization spectra to measured spectra of known C₄H₆ isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05), and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to C₄H₆ (<i>m</i>/<i>z</i> 54) + D and C₄H₅D (<i>m</i>/<i>z</i> 55) + H, formed at a ratio of 0.4 (<i>m</i>/<i>z</i> 54) to 1.0 (<i>m</i>/<i>z</i> 55). The D elimination channel forms almost exclusively 1,2-butadiene (0.97 ± 0.20) whereas the H elimination channel leads to the formation of deuterated 1,3-butadiene (0.89 ± 0.18) and 1-butyne (0.11 ± 0.02); photoionization spectra of undeuterated species are used in the fitting of the measured <i>m</i>/<i>z</i> 55 (C₄H₅D) spectrum. The results are generally consistent with a CH cycloaddition mechanism to the CC bond of propene, forming 1-methylallyl followed by elimination of a H atom via several competing processes. The direct detection of 1,3-butadiene as a reaction product is an important validation of molecular weight growth schemes implicating the CH + propene reaction, for example, those reported recently for the formation of benzene in the interstellar medium (Jones, B. M. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 452−457)