The thermal reactivity of HCN and NH3 in interstellar ice analogues

J. A. Noble, P. Theule, F. Borget, G. Danger, M. Chomat, F. Duvernay, F. Mispelaer, T. Chiavassa

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42 Scopus citations

Abstract

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.

Original languageEnglish
Pages (from-to)3262-3273
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Volume428
Issue number4
DOIs
StatePublished - Feb 1 2013
Externally publishedYes

Keywords

  • Astrochemistry
  • ISM: molecules
  • Methods: laboratory
  • Molecular data
  • Molecular processes

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