2007ApJ...662..758X

The importance of dust grains coated by water ice has been well recognized in astrophysical chemistry. Accordingly, the adsorption of various closed-shell species onto water ice has been much studied. However, the chemical behavior of free radicals (especially the molecular radicals) on water ice has attracted very little attention, either experimental or theoretical. In this paper, we for the first time investigate the adsorption and evolution of polycyanoacetylene radicals C_2n+1_ N (n=1-4) on both gaseous and ice water (only C₃ N for the latter) using computational methods. The reaction of C₃N with gaseous H₂ O seems unlikely to take place in low-temperature space, because of a minute energy barrier for hydrogen abstraction to form HC₃N plus OH via the adsorption complex H₂O⋅⋅⋅C₃N. Yet, this radical-molecule reaction can be effectively catalyzed by water ice. It is shown that the C_2n+1_ N (n=1-4) radicals possess much higher reactivity toward H₂ O than the simplest cyano (CN) radical or other well-known radicals. The adsorption of C_2n+1_ N radicals on water ice, simulated by (H₂O)₃ and (H₂O)₁₄, has two distinct outcomes in the low temperature of interstellar space. One is to be barrierlessly transformed into HC_2n+1_ N by capturing an H atom from water molecules near the water ice; the other is to be stabilized in the form of a C_2n+1_N⋅⋅⋅(H2_O)n_complex. This work may also represent the first study of water-catalyzed reactions associated with a molecular radical in space. The chemical behavior on water ice should have a prominent influence on the concentration of the polycyanoacetylene radicals C_2n+1_ N (n≥1), as well as their parents HC_2n+1_N, in space. Future experimental studies are highly desirable to probe such interesting processes.