Master equation modelling of the reactions of NH 2 with CH 2 O and NO Kevin M. Douglas 1 , Daniel Lucas 1 , Catherine Walsh 2 , Niclas A. West 1 , Mark A. Blitz 1 and Dwayne E. Heard 1 1 School of Chemistry, University of Leeds, UK, 2 School of Physics, University of Leeds, UK The reaction of NH 2 with CH 2 O has been suggested as a source of formamide (NH 2 CHO) in interstellar environments. To investigate this, we have used the Master Equation Solver for Multi-Energy well Reactions (MESMER) 1 to predict temperature and pressure dependent rate coefficients and branching ratios (BRs) for the reaction. The potential energy surface of NH 2 + CH 2 O system was calculated at the CCSD(T)//M062X-aug- cc-pVTZ level of theory using the Gaussian 09 suit of programs.The reaction may proceed via the formation of one of two pre-reaction complexes (PRCs), from which there are two exothermic product channels; a hydrogen- abstraction channel in which the NH 2 abstracts an H atom from formaldehyde to produce ammonia, NH 3 , and the formyl radical, CHO (R1a), and an addition-elimination channel in which the NH 2 first attacks the C of the formaldehyde to form a bound adduct, which then goes on to eliminate an H atom and produce formamide + H (R1b): NH 2 + CH 2 O → NH 3 + CH 2 O (R1a) - 79 kJ mol -1 → NH 2 CHO + H (R1b) - 45 kJ mol -1 MESMER modelling of this reaction indicates that at the low temperatures of the interstellar medium, the lifetimes of the weakly bound PRCs are extended, and as such unimolecular decay back to reactants competes with reactive removal (either over a barrier or through it via tunnelling) to products. Stabilization of the PRCs can also occur via collisions with a third body; as such, the reaction exhibits a strong pressure dependence. We have also investigated the reaction between NH 2 and NO with MESMER, using a PES surface calculated at the B3LYP/6-311G(d,p) level of theory. The NH 2 + NO PES is complex, with many deep wells and large barriers, and three possible product channels: NH 2 + NO → N 2 + H 2 O (R2a) - 468 kJ mol -1 → N 2 O + H 2 (R2b) - 192 kJ mol -1 → HN 2 + OH (R2c) + 7.5 kJ mol -1 The reaction is initiated by the barrierless addition of the NH 2 to the NO to form the adduct H 2 NNO. This adduct undergoes an H-atom shift followed by a series of cis-trans isomerizations, giving rise to four distinct HNNOH isomers. All four of these isomers may undergo unimolecular decay to form the products HN 2 + OH (R2a), while only one of the isomers is configured correctly to form the N 2 + H 2 O products (R2c) via a four membered ring transition state. Stabilisation into the deep wells requires very high pressures (> 1 × 10 22 molecules cm -3 ), and as such the predicted rate coefficients and BRs are effectively pressure independent at atmospheric pressure and below. The BRs do however heavily depend on the reverse inverse Laplace transform parameters (for the unimolecular decay of the HNNOH isomers) used in MESMER; at higher temperatures, this unimolecular decay is favourable and as such the endothermic HN 2 + OH channel is dominant, while at low temperatures, the low energy route to N 2 + H 2 O is favoured. References 1. Glowacki, D. R.; Liang, C.-H.; Morley, C.; Pilling, M. J.; Robertson, S. H., MESMER: An Open-Source Master Equation Solver for Multi-Energy Well Reactions. The Journal of Physical Chemistry A 2012, 116 (38), 9545-9560.
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