The thermal reactivity of HCN and NH₃ in interstellar ice analogues

HCN is a molecule central to interstellar chemistry, since it is the simplestmolecule containing a carbon-nitrogen bond and its solid state chemistry is rich. The aim of this work was to study the NH₃ + HCN → NH⁺₄ CN^- thermal reaction in interstellar ice analogues. Laboratory experiments based on Fourier transform infrared spectroscopy and mass spectrometry were performed to characterize the NH⁺₄ 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 ACN- = 1.8 ± 1.5 × 10-17 cm molecule-1 at both 20 and 140 K. NH⁺₄ CN- exhibits zeroth-order multilayer desorption kinetics with a rate of kdes(T) = 1028 molecule cm^-2 s^- exp(-38.0±1.4 kJmol^- RT). The NH3 + HCN→NH⁺₄ CN^- thermal reaction is of primary importance because (i) it decreases the amount of HCN available to be hydrogenated into CH2NH, (ii) the NH⁺₄ and CN^- ions react with species such as H2CO and CH2NH to form complex molecules and (iii) NH⁺₄ CN^- is a reservoir of NH₃ and HCN, which can be made available to a high-temperature chemistry.