Kinetic Study of the Gas-Phase Reaction between Atomic Carbon and Acetone. Low Temperature Rate Constants and Hydrogen Atom Product Yields

Abstract

The reactions of ground state atomic carbon, C(3P), are likely to be important in astrochemistry due to the high abundance levels of these atoms in the dense interstellar medium. Here we present a study of the gas-phase reaction between C(3P) and acetone, CH3COCH3. Experimentally, rate constants were measured for this process over the 50 to 296 K range using a continuous-flow supersonic reactor, while secondary measurements of H(2S) atom formation were also performed over the 75 to 296 K range to elucidate the preferred product channels. C(3P) atoms were generated by In-situ pulsed photolysis of carbon tetrabromide, while both C(3P) and H(2S) atoms were detected by pulsed laser induced fluorescence. Theoretically, quantum chemical calculations were performed to obtain the various complexes, adducts and transition states involved in the C(3P) + CH3COCH3 reaction over the 3A'' potential energy surface, allowing us to better understand the reaction pathways and help to interpret the experimental results. The derived rate constants are large, (2-3) x 10-10 cm3 s-1 , displaying only weak temperature variations; a result that is consistent with the barrierless nature of the reaction. As this reaction is not present in current astrochemical networks, its influence on simulated interstellar acetone abundances is tested using a gas-grain dense interstellar cloud model. For interstellar modelling purposes, the use of a temperature independent value for the rate constant, k(C+CH3COCH3 )= 2.2 x 10-10 cm3 s-1, is recommended. The C(3P) + CH3COCH3 reaction decreases gas-phase CH3COCH3 abundances by as much as two orders of magnitude at early and intermediate cloud ages.