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Chomat, M.
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article
The thermal reactivity of HCN and NH<SUB>3</SUB> in interstellar ice analogues
Abstract
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 NH<SUB>3</SUB> + HCN → NH<SUP>+</SUP><SUB>4</SUB>CN<SUP>-</SUP> thermal reaction in interstellar ice analogues. Laboratory experiments based on Fourier transform infrared spectroscopy and mass spectrometry were performed to characterize the NH<SUP>+</SUP><SUB>4</SUB>CN<SUP>-</SUP> 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<SUP>+ 0.010</SUP><SUB>- 0.006</SUB> s^{-1}exp (-2.7± 0.4 text{kJ mol}^{-1}/RT) when NH<SUB>3</SUB> is much more abundant than HCN. When both reactants are diluted in water ice, the reaction is slowed down. We have estimated the CN<SUP>-</SUP> ion band strength to be A_{text{CN}^-} = 1.8 ± 1.5 × 10<SUP>-17</SUP> cm molecule<SUP>-1</SUP> at both 20 and 140 K. NH<SUP>+</SUP><SUB>4</SUB>CN<SUP>-</SUP> exhibits zeroth-order multilayer desorption kinetics with a rate of k_{text{des}}(T) = 10<SUP>28</SUP> molecule cm<SUP>-2</SUP> s^{-1}exp (-38.0± 1.4text{ kJ mol}^{-1}/RT). The NH<SUB>3</SUB> + HCN → NH<SUP>+</SUP><SUB>4</SUB>CN<SUP>-</SUP> thermal reaction is of primary importance because (i) it decreases the amount of HCN available to be hydrogenated into CH<SUB>2</SUB>NH, (ii) the NH<SUP>+</SUP><SUB>4</SUB> and CN<SUP>-</SUP> ions react with species such as H<SUB>2</SUB>CO and CH<SUB>2</SUB>NH to form complex molecules and (iii) NH<SUP>+</SUP><SUB>4</SUB>CN<SUP>-</SUP> is a reservoir of NH<SUB>3</SUB> and HCN, which can be made available to a high-temperature chemistry....