Astronomy and Astrophysics – Astronomy
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006apj...643..573x&link_type=abstract
The Astrophysical Journal, Volume 643, Issue 1, pp. 573-581.
Astronomy and Astrophysics
Astronomy
5
Astrochemistry, Ism: Molecules, Molecular Processes
Scientific paper
Reactions of atoms and molecules on dust grains coated by water ice have been considered to play an important role in the formation of many of the detected interstellar molecules. The catalytic role of water ice in some molecule-molecule reactions has been shown. Yet, there is rather little knowledge on the water-catalyzed atom-molecule reactions. In this paper, we present an evident theoretical clue that a considerably barrier-consumed atom-molecule reaction 3O+HC3N in the gas phase can be effectively catalyzed by water ice to be barrierless. Detailed Gaussian-3 potential energy surface studies show that both the 3O+HC3N and 1O+HC3N reactions in the gas phase can eventually lead to the product 3HCCN+CO with the involvement of singlet-triplet intersystem crossings. Yet the gaseous 3O+HC3N reaction faces a considerable entrance barrier, suggestive of its low likelihood in the low-temperature interstellar space. Fortunately, inclusion of water molecules significantly reduces the entrance barrier to zero. This is caused by the dual role of water molecules in the rate-determining O-addition transition state for 3O+HC3N, i.e., the water molecules act as both a proton acceptor with the CH bond and a proton donor with the 3O atom to cooperatively decrease the reaction barrier. The results can well account for the recent cyanoketene detection in a photoreactivity study of HC3N/O3/Ar mixtures. Therefore, on water ice, the simplest cyanopolyynes, HC3N, can be effectively depleted by oxygen atoms and can generate the elusive cyanomethylene (3HCCN), whose formation mechanism in space was still unclear up to now.
Ding Yi-hong
Sun Chia-chung
Xie Hong-bin
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