The Gas-phase Reaction of NH<sub>2</sub> with Formaldehyde (CH<sub>2</sub>O) is not a Source of Formamide (NH<sub>2</sub>CHO) in Interstellar Environments
Kevin M. Douglas, Daniel I. Lucas, Catherine Walsh, Niclas A. West, Mark A. Blitz, Dwayne E. Heard
Abstract
Abstract The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH 2 with formaldehyde (CH 2 O) has been performed. This reaction has previously been suggested as a source of formamide (NH 2 CHO) in interstellar environments. A pulsed Laval nozzle equipped with laser-flash photolysis and laser-induced fluorescence spectroscopy was used to create and monitor the temporal decay of NH 2 in the presence of CH 2 O. No loss of NH 2 could be observed via reaction with CH 2 O, and we place an upper limit on the rate coefficient of <6 × 10 −12 cm 3 molecule −1 s −1 at 34 K. Ab initio calculations of the potential energy surface were combined with Rice–Rampsberger–Kassel–Marcus (RRKM) calculations to predict a rate coefficient of 6.2 × 10 −14 cm 3 molecule −1 s −1 at 35 K, consistent with the experimental results. The presence of a significant barrier, 18 kJ mol −1 , for the formation of formamide as a product, means that only the H-abstraction channel producing NH 3 + CHO, in which the transfer of an H atom can occur by quantum mechanical tunneling through a 23 kJ mol −1 barrier, is open at low temperatures. These results are in contrast with a recent theoretical study, which suggested that the reaction could proceed without a barrier and was therefore a viable route to gas-phase formamide formation. The calculated rate coefficients were used in an astrochemical model, which demonstrated that this reaction produces only negligible amounts of gas-phase formamide under interstellar and circumstellar conditions. The reaction of NH 2 with CH 2 O is therefore not an important source of formamide at low temperatures in interstellar environments.