Abstract
Photochemically driven reactions involving unsaturated radicals produce a thick global layer of organic haze on Titan, Saturn’s largest moon. The allyl radical self-reaction is an example for this type of chemistry and was examined at room temperature from an experimental and kinetic modelling perspective. The experiments were performed in a static reactor with a volume of 5 L under wall free conditions. The allyl radicals were produced from laser flash photolysis of three different precursors allyl bromide (C3H5Br), allyl chloride (C3H5Cl), and 1,5-hexadiene (CH2CH(CH2)2CHCH2) at 193 nm. Stable products were identified by their characteristic vibrational modes and quantified using FTIR spectroscopy. In addition to the (re-) combination pathway C3H5+C3H5 → C6H10 we found at low pressures around 1 mbar the highest final product yields for allene and propene for the precursor C3H5Br. A kinetic analysis indicates that the end product formation is influenced by specific reaction kinetics of photochemically activated allyl radicals. Above 10 mbar the (re-) combination pathway becomes dominant. These findings exemplify the specificities of reaction kinetics involving chemically activated species, which for certain conditions cannot be simply deduced from combustion kinetics or atmospheric chemistry on Earth.
Highlights
The allyl radical (C3H5) is the smallest π-conjugated radical
Modeling studies show that the knowledge of accurate kinetic data for the reactions of unsaturated C3-species is essential for the description of reactive particle formation processes [4,9,10,12,13]
The product formation in the allyl radical self-reaction was examined at room temperature in laser flash photolysis experiments using the precursors C3H5Br, C3H5Cl, and 1,5-hexadiene (C6H10)
Summary
The allyl radical (C3H5) is the smallest π-conjugated radical. Its reactions with other hydrocarbon radicals are a building block for basic kinetic schemes for hydrocarbon combustion and for photochemical models of the planetary atmospheres [1,2,3,4]. Modeling studies show that the knowledge of accurate kinetic data for the reactions of unsaturated C3-species is essential for the description of reactive particle formation processes [4,9,10,12,13]. For assessing the role of the allyl chemistry in complex reactive systems it is crucial to characterize the rates of allyl-consuming reactions, and the product branching in the respective pathways and its pressure dependency. Both the formation of secondary organic aerosols on
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