Astronomy and Astrophysics – Astronomy
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.294p&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Astronomy and Astrophysics
Astronomy
Scientific paper
The question of the relative stability of the complex organic molecules (COM) under the interstellar radiation field is a crucial question, especially in the context of the panspermia hypothesis for which their survival during the transfer from space to the Earth is a necessary condition for the appearance of life (Ehrenfreund et al 2001, 2002). Assuming that these species are originally embedded in interstellar grains, their resistance to the solar UV radiation in ice is a key issue. The case of glycine, H2NCH2COOH, has been considered recently in irradiation experiments carried out at the SOLEIL synchrotron facility. It appeared that glycine is partially protected by ice but also suffers some fragmentation leading in the end to CO2 and to CN containing species. Quantum chemical simulations have been performed in parallel for all the possible fragmentations of neutral, ionized, doubly ionized, protonated and zwitterionic glycine (Lattelais et al. 2010). This study has shown that the primary decomposition routes leading to fragments CH2COOH, H2NCH2, H2NCH2+, CH2COOH+, H2NCH3, and CO2 are energetically valid. The experiments have also shown that the role of water is mainly to increase the production of the end products in the photoreactions, most probably due to the reactions with the OH radicals formed during the irradiation of the ice. We present a theoretical study of the secondary fragmentation channels initiated by the OH radical by means of quantum chemical calculations using DFT and ab initio correlated methods (MP2,CCSD(T)). We have examined all the possible secondary fragmentations starting from the primary fragments mentioned above. We considered all species embedded in the dielectric continuum simulating an icy environment (comparison with the gas phase situation serves as reference). Thermodynamic stabilities as well as activation barriers have been determined. This study shows that the formation of the HCN molecule as end product is energetically possible and that the secondary fragmentations are effectively more efficient in presence of water ice, which supports the previous experimental conclusions. Our results suggest also that the OH radical is a very reactive species in the ice and could be involved in a large number of processes relevant to the mantle chemistry of interstellar grains (see communication by Redondo et al).
Ellinger Yves
Pernet A.
Pilme J.
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