The gas-phase reaction of NH₂ with formaldehyde (CH₂O) is not a source of formamide (NH₂CHO) in interstellar environments

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH₂ with formaldehyde (CH₂O) has been performed. This reaction has previously been suggested as a source of formamide (NH₂CHO) 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 NH₂ in the presence of CH₂O. No loss of NH₂ could be observed via reaction with CH₂O, and we place an upper limit on the rate coefficient of <6 × 10¯¹² cm³ 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 cm³ 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 NH₃ + 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 NH₂ with CH₂O is therefore not an important source of formamide at low temperatures in interstellar environments.