The thermal reactivity of HCN and NH3 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 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.

12 pages, 9 figures, 3 tables. Accepted for publication in MNRAS

References in corpus (4)

• Thermal desorption characteristics of CO, O2 and CO2 on non-porous water, crystalline water and silicate surfaces at sub-monolayer and multilayer coverages
• Desorption of CO and O2 interstellar ice analogs
• Chemistry in Disks. V: CN and HCN in proto-planetary disks
• Hydrogen cyanide and isocyanide in prestellar cores

Cited by in corpus (24)

• An Ice Age JWST inventory of dense molecular cloud ices
• Atom addition reactions in interstellar ice analogues
• Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?
• Hydrogenation at low temperatures does not always lead to saturation: the case of HNCO
• Evidence of surface catalytic effect on cosmic dust grain analogues: the ammonia and carbon dioxide surface reaction
• Ammonium hydrosulfide (NH4SH) as a potentially significant sulfur sink in interstellar ices
• Ammonia snow-lines and ammonium salts desorption
• Multiple paths of deuterium fractionation in protoplanetary disks
• The ALMA-PILS survey: Gas dynamics in IRAS 16293$-$2422 and the connection between its two protostars
• Kinetics and mechanisms of the acid-base reaction between NH${}_3$ and HCOOH in interstellar ice analogs
• Influence of surface and bulk water ice on the reactivity of a water-forming reaction
• Investigation of HNCO isomers formation in ice mantles by UV and thermal processing: an experimental approach
• Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying deuteration at high angular resolution toward protoplanetary disks
• First in-situ detection of the CN radical in comets and evidence for a distributed source
• Dust-gas chemistry in AGB outflows: Chemical modelling of dust-gas chemistry within AGB outflows I. Effect on the gas-phase chemistry
• The nitrogen carrier in protoplanetary disks
• The effect of selective desorption mechanisms during interstellar ice formation
• Predicting binding energies of astrochemically relevant molecules via machine learning
• Chemical signatures of a warped protoplanetary disc
• Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments
• The formation of glycolonitrile (HOCH${}_2$CN) from reactions of C${}^{+}$ with HCN and HNC on icy grain mantles
• The Molecular Composition of Shadowed Protosolar Disk Midplanes beyond the Water Snowline
• Low NH${}_3$/H${}_2$O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun
• HCN snowlines in protoplanetary disks: constraints from ice desorption experiments