The thermal reactivity of HCN and NH3 in interstellar ice analogues
  HCN is a molecule central to interstellar chemistry, since it is the simplest
molecule containing a carbon-nitrogen bond and its solid state chemistry is
rich. The aim of this work was to study the NH3 + HCN -> NH4+CN- thermal
reaction in interstellar ice analogues. Laboratory experiments based on Fourier
transform infrared spectroscopy and mass spectrometry were performed to
characterise the NH4+CN- reaction product and its formation kinetics. This
reaction is purely thermal and can occur at low temperatures in interstellar
ices without requiring non-thermal processing by photons, electrons or cosmic
rays. The reaction rate constant has a temperature dependence of k(T) =
0.016+0.010-0.006 s-1.exp((-2.7+-0.4 kJmol-1)/(RT)) when NH3 is much more
abundant than HCN. When both reactants are diluted in water ice, the reaction
is slowed down. We have estimated the CN- ion band strength to be A_CN- =
1.8+-1.5 x10-17 cm molec-1 at both 20 K and 140 K. NH4+CN- exhibits
zeroth-order multilayer desorption kinetics with a rate of k_des(T) = 10^28
molecules cm-2 s-1.exp((-38.0+-1.4 kJmol-1)/(RT)). The NH3 + HCN -> NH4+CN-
thermal reaction is of primary importance because (i) it decreases the amount
of HCN available to be hydrogenated into CH2NH, (ii) the NH4+ and CN- ions
react with species such as H2CO, or CH2NH to form complex molecules, and (iii)
NH4+CN- is a reservoir of NH3 and HCN, which can be made available to a high
temperature chemistry.
