Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007apj...662..758x&link_type=abstract
The Astrophysical Journal, Volume 662, Issue 1, pp. 758-770.
Astronomy and Astrophysics
Astronomy
1
Astrochemistry, Ism: Dust, Extinction, Ism: Molecules
Scientific paper
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 C2n+1N (n=1-4) on both gaseous and ice water (only C3N for the latter) using computational methods. The reaction of C3N with gaseous H2O 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 C2n+1N (n=1-4) radicals possess much higher reactivity toward H2O than the simplest cyano (CN) radical or other well-known radicals. The adsorption of C2n+1N 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 HC2n+1N by capturing an H atom from water molecules near the water ice; the other is to be stabilized in the form of a C2n+1N...(H2O)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 C2n+1N (n>=1), as well as their parents HC2n+1N, in space. Future experimental studies are highly desirable to probe such interesting processes.
Ding Yi-hong
Shi Guo-Sheng
Xie Hong-bin
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