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 -> NH +4CN- thermal reaction in interstellar ice analogues. Laboratory experiments based on Fourier transform infrared spectroscopy and mass spectrometry were performed to characterize the NH +4CN- 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(-0.006)(+0.010) s(-1) exp(-2.7 +/- 0.4 kJ mol(-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 x 10(-17) cm molecule(-1) at both 20 and 140 K. NH +4CN- exhibits zeroth-order multilayer desorption kinetics with a rate of k(des)(T) = 10(28) molecule cm(-2) s(-1) exp(-38.0 +/- 1.4 kJ mol(-1)/RT). The NH3 + HCN -> NH +4CN- 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 and CH2NH to form complex molecules and (iii) NH +4CN- Nis a reservoir of NH3 and HCN, which can be made available to a high-temperature chemistry.