Studying the kinetics of the gas-phase reaction of NH 2 with acetone at very low temperatures using experimental and theoretical methods Ffion Hall , Kevin Douglas, Alice Kirker and Dwayne Heard School of Chemistry, University of Leeds, UK There have now been over 270 molecules detected in the interstellar medium (ISM) 1,2 , ranging from small radicals to complex organic molecules (COMs). Amongst these COMs include nitrogen-containing molecules, for example acetamide, CH 3 C(O)NH 2 . How such molecules are formed in the ISM, where temperatures can be as low as just 10 K 3 , is a pressing question in the field of astrochemistry, and one that studying the kinetics of the gas-phase reactions of NH 2 with COMs could help to answer. In this work, the gas-phase reaction between NH 2 and acetone was studied theoretically using ab initio methods to construct a potential energy surface and transition state theory to calculate theoretical temperature and pressure-dependent rate coefficients and product branching ratios. The theoretical results were complimented by experimental data obtained using a pulsed Laval nozzle apparatus which can achieve a minimum temperature of 30 K, in combination with pulsed laser photolysis (PLP) and laser flash photolysis (LFP). A hydrogen abstraction pathway to form NH 3 + CH 2 C(O)CH 3 and an addition-elimination pathway to form CH 3 C(O) NH 2 + CH 3 were both considered. Theoretical results showed hydrogen abstraction to be dominant owing to the presence of a van der Waals pre-reaction complex that can tunnel quantum mechanically through the activation barrier at low temperatures 4,5 and hence a strong negative temperature dependence and pressure was revealed. Experimental loss rates of NH 2 were in the region of ~10 -11 molecule -1 cm 3 s -1 , suggesting the reaction proceeds at a fast enough rate to be of astrochemical significance in terms of observed abundances of NH 2 and acetone in the ISM, as well as the potential exciting chemistry of the CH 2 C(O)CH 3 radical product. References 1. Cologne, U.o. Molecules in Space. 2020.[accessed 24th January 2023]; Available from: https://cdms.astro.uni-koeln.de/ classic/molecules?do. 2. Wooten, A., ALMA capabilities for observations of spectral line emission. Astrophysics and Space Science, 2008. 313 : pp. 9-12. 3. Herbst, E., The synthesis of large interstellar molecules. International Reviews in Physical Chemistry, 2017. 36 (2): pp. 287- 331. 4. Shannon, R.J., Blitz, M. A., Goddard, A., Heard, D. E., Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling. Nature Chemistry, 2013. 5 : pp. 745-749. 5. Heard, D.E., Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunnelling. Accounts of Chemical Research, 2018. 51 : pp. 2620-2627.
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