The Complex (Organic) Puzzle of the Formation of Hydrogen Cyanide and Isocyanide on Interstellar Ice Analogues

Abstract

Aiming to constrain the surface formation of HCN and HNC in the dense interstellar medium on ice-covered dust grains, we investigate the interaction of CN radicals with H₂O and CO ices and their subsequent reactivity with H and H₂. CN radicals can physisorb on both ices. However, on H₂O ice, a hemibond formation is the most common binding mode, while on CO ice, the CN-CO van der Waals complex can form NCCO with a small energy barrier. We show low barrier or barrierless pathways to the formation of HCN and HNC for the reaction H + CN on both ices. Reactivity with H₂ involves activation energy barriers to form HCN, which may be overcome by quantum tunneling, while HNC formation is unlikely. The formation of HCN and HNC competes with the formation of NH₂CHO on H₂O and HCOCN on CO.

Figure 1

Binding energy distributions of CN on water ices. There are 26 and 27 unique sites from (H₂O)₁₄ and (H₂O)₁₈ ice models. All energies are corrected for zero point energies. The insets correspond to representative cases from the distribution, one for H-bonding and another for hemibonding, showing also the electron spin density isosurface (isovalue = 0.005 au).

Figure 2

Scaled binding energy distribution of CN on CO ices. There are 12 and 22 unique instances from (CO)₁₃ and (CO)₁₈ ice models. All energies are corrected for zero point energies. Notice that in most cases we could not get rid of spurious imaginary frequencies, see the main text for a discussion. An example of a binding geometry is shown in the inset. This inset also contains the isosurface of the spin-density (isovalue = 0.005 au).

Figure 3

Rate constants for NCCO formation and H₂ + CN in three binding modes (see SI for further details). The subscripts “VdW”, “H-bonded”, and “hemi” stand for van der Waals, hydrogen bonded, and hemibonded, respectively.