  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
<6x10-12 cm3 molecule-1 s-1 at 34K. Ab initio calculations of the potential
energy surface were combined with RRKM calculations to predict a rate
coefficient of 6.2x10-14 cm3 molecule-1 s-1 at 35K, 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 tunnelling 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.
