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Can cyanide radicals drive molecular backbone growth on interstellar icy grains?

Germán Molpeceres, Joan Enrique-Romero

Abstract

Motivated by the value of CN-bearing molecules as tracers of interstellar physical conditions, we investigate the reactions of adsorbed CN radicals with acetylene and ethylene (C2H2 and C2H4) on interstellar dust-grain analogues using quantum chemical calculations. We find that reactivity is strongly controlled by the relative orientation of the reactants, with specific geometries either promoting or inhibiting reaction. We further show that, on ice, these reactions differ qualitatively from their gas-phase counterparts, stalling at the formation of the adduct complexes C2H2CN and C2H4CN and exhibiting newly emerged kinetic barriers for the neutral-radical association. We contextualize our calculations in the same reaction-diffusion framework that would be employed in astrochemical models, finding that, depending on the diffusion energy of the hydrocarbons, these reactions can be either negligible or efficient, highlighting the importance of the local ice structure in interstellar grain chemistry. These findings caution against the use of CN-based tracers that assume barrierless, bimolecular surface reactions involving CN radicals.

Can cyanide radicals drive molecular backbone growth on interstellar icy grains?

Abstract

Motivated by the value of CN-bearing molecules as tracers of interstellar physical conditions, we investigate the reactions of adsorbed CN radicals with acetylene and ethylene (C2H2 and C2H4) on interstellar dust-grain analogues using quantum chemical calculations. We find that reactivity is strongly controlled by the relative orientation of the reactants, with specific geometries either promoting or inhibiting reaction. We further show that, on ice, these reactions differ qualitatively from their gas-phase counterparts, stalling at the formation of the adduct complexes C2H2CN and C2H4CN and exhibiting newly emerged kinetic barriers for the neutral-radical association. We contextualize our calculations in the same reaction-diffusion framework that would be employed in astrochemical models, finding that, depending on the diffusion energy of the hydrocarbons, these reactions can be either negligible or efficient, highlighting the importance of the local ice structure in interstellar grain chemistry. These findings caution against the use of CN-based tracers that assume barrierless, bimolecular surface reactions involving CN radicals.
Paper Structure (13 sections, 9 equations, 7 figures)

This paper contains 13 sections, 9 equations, 7 figures.

Table of Contents

  1. Introduction
  2. Computational methods
  3. Results
  4. Reaction coordinate sampling. Energetics of the CN + C$_{\rm X}H_{\rm Y}$ reactions
  5. Reaction of hemibonded CN with H2O ice
  6. Kinetic analysis: Reaction-diffusion competition
  7. Discussion
  8. Integration of the results into astrochemical models
  9. Implications for CN driven astrochemistry
  10. Conclusion
  11. Conversion of H-bond to hemibonded CN in a 14 H2O water cluster
  12. Eley-Rideal mechanism
  13. Nature of the CN + C$_{\rm X}$H$_{\rm Y}$ reactions on ices

Figures (7)

  • Figure 1: Inverse of diffusion characteristic times for C_XH_Y and CN, shown as a function of dust temperature and diffusion energy. The color scale is saturated outside the range $10^{-6}$–$10^{6}$ s$^{-1}$; values represented with the extreme colors may therefore correspond to characteristic times significantly smaller or larger than these limits. An attempt frequency of $10^{12}$ s$^{-1}$ is assumed hasegawa_three-phase_1993.
  • Figure 2: (Left) Depiction of the hemibonded CN radical adsorbed on a 14 H2O cluster. (Right, Top) Parallel geometries for the approach of C2H2 and C2H4 to hemibonded CN. (Right, Bottom) Collinear geometries for the approach of C2H2 and C2H4 to hemibonded CN. All the structures are optimized and represent the actual reactant states used in the determination of the energetic descriptors of the reaction.
  • Figure 3: Reaction profiles for the CN + C_XH_Y reaction in our ice analogues. (Top) CN + C2H2. (Bottom) CN + C2H4.
  • Figure 4: (Top) Geometry of the transition state corresponding to the reaction with the water matrix in our setup (Bottom) Reaction rate constant for the reaction using our derived activation energy (8.5 $\mathrm{kcal\,mol^{-1}}$) and Rimola2018 one (3.5 $\mathrm{kcal\,mol^{-1}}$). We note that in the calculation of the rate constant only the activation energy was modified, with the vibrational and tunneling contributions coming exclusively from our calculations (see text). The gray box represent the H-accretion competing process, assumed to be 1 atom day$^{-1}$. The data used to generate this figure can be obtained as data behind the figure.
  • Figure 5: Reaction with C_XH_Y (Top C2H2 and bottom C2H4), diffusion, desorption and reaction with water efficiencies obtained as a function of $E_{\rm diff}$. Note that CN diffusion and desorption are neglected. The data to reproduce the leftmost column can be retrieved as data behind the figure.
  • ...and 2 more figures