Branching ratios in the hydroxyl reaction with propene

The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N₂) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water H₂¹⁸O was used to produce ¹⁸O-labeled hydroxyl radicals in reaction with fluorine atoms. The β-hydroxypropyl radicals formed in the attachment reactions 1aand 1b, OH + C₃H₆ → CH₂(OH) C•HCH₃ (eq 1a) and OH + C₃H₆ → C•H₂CH(OH)CH₃ (eq 1b), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O₂- and NO-containing environment. The ¹⁸O-labeling propagates to the final products, allowing determination of the branching ratio for the attachment channels of reaction 1. The measured branching ratio for attachment is β_1b = k_1b/(k_1a + k_1b) = 0.51 ± 0.03, independent of pressure over the 60-400 Torr pressure range. An upper limit on the hydrogen-abstraction channel, OH + C₃H₆ → H₂O + C₃H₅ (eq 1c), was determined by measuring the water yield in reactions of OH and OD radicals (produced via H(D) + NO₂ → OH(OD) + NO reactions) with C₃H₆ as k_1c/(k_1a + k_1b + k_1c) < 0.05 (at 298 K, 200 Torr N₂).