Low-temperature study of the reactions of NH 2 with formaldehyde, acetaldehyde, and NO Kevin M. Douglas 1 , Daniel Lucas 2 , Lok H. D. Li 1 , Niclas West 1 Mark Blitz 1,3 , Catherine Walsh 4 , Dwayne Heard 1,3 1 School of Chemistry, University of Leeds, LS2 9JT, UK, 2 School of Chemistry, University of Birmingham, B15 2TT, UK, 3 National Centre for Atmospheric Science, University of Leeds, LS2 9JT, UK, 4 School of Physics and Astronomy, University of Leeds, LS2 9JT, UK A major open question in astrochemistry concerns the mechanisms for the formation of complex organic molecules, with particular interest in those molecules that may play a role in prebiotic chemistry. Formamide (NH 2 CHO) and acetamide (NH 2 COCH 3 ) are two such molecules, as both contain the peptide bond (HN-C=O), the type of bond that plays a key role in linking amino acids into proteins. The reaction between NH 2 + formaldehyde (CH 2 O) has been suggested as possible source of formamide in the interstellar medium (ISM), while the analogous reaction between NH 2 + acetaldehyde (CH 3 CHO) may also be a source of acetamide. To investigate this, an experimental, theoretical, and modelling study into the reactions between NH 2 + CH 2 O and CH 3 CHO has been conducted. Experiments were carried out using a pulsed laser photolysis-laser induced fluorescence technique in which the loss of NH 2 radicals in the presence of CH 2 O or CH 3 CHO were monitored. Low-temperatures relevant to the ISM were achieved using a pulsed Laval nozzle expansion. No loss of NH 2 could be observed via reaction with CH 2 O, indicating that the reaction is slow even at low temperatures. Ab initio calculations of the potential energy surface (PES) of the NH 2 + CH 2 O system were combined with RRKM calculations to predict rate coefficients and branching ratios (BRs) over a broad range of temperatures and pressures. The presence of significant barrier, 18 kJ mol -1 , for the formation of the formamide product, means that only the H-abstraction channel forming NH 3 + CHO, in which the transfer of an H-atom can occur by quantum mechanical tunnelling through a 23 kJ mol -1 barrier is open at low temperatures. In contrast, we observer NH 2 removal by acetaldehyde over a range of temperatures (28 – 107 K) in our experiments, with the reaction showing a positive pressure dependence and a negative temperature dependence. Theoretical modelling of this reaction indicates that the loss of NH 2 we observe in our experiments is primarily due to the formation of two weakly bound pre-reaction complexes. However, product studies observing the formation of CH 3 CO ( via its titration to OH with O 2 ) do indicate a small fraction of chemical reaction occurring, producing the H-abstraction products NH 3 + CH 3 CO. We have also investigated the low-temperature reaction between NH 2 + NO, an important reaction in the thermal deNOx process. Experiments monitoring the loss of NH 2 in the presence of NO were conducted at temperatures ranging from 24 to 106 K. Our results are in agreement with higher temperature data at 200 K and above, showing that the reaction exhibits a strong negative temperature dependence, reaching a rate coefficient of 3.4 × 10 -10 cm 3 molecule -1 s -1 at 24 K. The reaction has also been studied theoretically.ThePES is complex, with many deep wells and large barriers, and three possible product channels. Rate coefficients and BRs predicted using the PES indicate that at low temperatures, only the low energy route to N 2 + H 2 O is open, while at higher temperatures the endothermic HN 2 + OH channel becomes favourable.
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