Effect of temperature on the gas-phase reaction of CH 3 CN with OH radicals: experimental ( T = 11.7-177.5 K) and computational ( T = 10-400 K) kinetic study.

Acetonitrile (CH 3 CN) is present in the interstellar medium (ISM) in a variety of environments. However, at the ultracold temperatures of the ISM, radical-molecule reactions are not widely investigated because of the experimental handicap of getting organic molecules in the gas phase by conventional techniques. The CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique solves this problem. For this reason, we present in this work the kinetic study of the gas-phase reaction of CH 3 CN with one of the most ubiquitous radicals, the hydroxyl (OH) radical, as a function of temperature (11.7-177.5 K). The kinetic technique employed to investigate the CH 3 CN + OH reaction was the pulsed laser photolysis-laser induced fluorescence. The rate coefficient for this reaction k ( T ) has been observed to drastically increase from 177.5 K to 107.0 K (about 2 orders of magnitude), while the increase in k ( T ) from 107.0 K to 11.7 K was milder (around 4 times). The temperature dependent expressions for k ( T ) are provided in the two distinct T -ranges, excluding the upper limit obtained for k (177.5 K): In addition, the rate coefficients estimated by the canonical competitive unified statistical (CCUS) theory show a similar behaviour to the experimental results, when evaluated within the high-pressure limit. This is consistent with the experimentally observed independence of k ( T ) with total gas density at selected temperatures. Astrochemical networks, such as the KIDA database or UMIST, do not include the CH 3 CN + OH reaction as a potential depletion process for acetonitrile in the ISM because the current studies predict very low rate coefficients at IS temperatures. According to the model ( T = 10 K), the impact of the titled reaction on the abundances of CH 3 CN appears to be negligible in dark molecular clouds of the ISM (∼1% of the total depletion reactions included in UMIST network). With respect to the potential formation of the CH 2 CN radical in those environments, even in the most favourable scenario, where this radical could be formed in a 100% yield from the CH 3 CN + OH reaction, this route would only contribute around 2% to the current assumed formation routes by the UMIST network.