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Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p..95b&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Other
Scientific paper
The fact that, for most of the species observed so far in the interstellar medium, the most abundant isomer of a given generic formula is the most stable one (Minimum Energy Principle MEP) suffers very few exceptions. Among them, two couples of isomers CH_3COOH/HCOOCH_3 and CH_3CH_2 OH/CH_3OCH_3 whose formation is thought to occur on the icy mantles of interstellar grains. In this context, it is therefore important to know whether a selective adsorption at the ice surface is able to monitor the gas phase abundance of one isomer with respect to the other. To this purpose, we present a coherent and concerted theoretical/experimental study of the adsorption energies of the four molecules mentioned above, i.e. acetic acid (AA)/methyl formate (MF) and ethanol (EtOH)/dimethyl ether (DME) on the surface of crystalline water ice at low temperature. The theoretical approach was addressed at LCT using solid state periodic DFT to represent the organized solid support while the experimental investigations of the molecule/ice interactions was carried out independently by two groups at LPMAA and LAMAp/LERMA using Temperature Programmed Desorption (TPD) under ultra-high vacuum (UHV) between 70 and 160 K. For each pair of isomers, theory and experiments both agree that the most stable isomer (AA or EtOH) interacts more efficiently with the water ice than the higher energy isomer (MF or DME). This differential adsorption shows clearly in the different desorption temperatures of the isomers. It is not related to their intrinsic stability but linked to the fact that both AA and EtOH make more and stronger hydrogen bonds with the ice surface. In conclusion, the present study shows that hydrogen bonding can be a factor playing an important role in the release of organics species from grains and suggest that differential adsorption could contribute to interpret MEP exceptions.
Baouche Saoud
Bertin Mario
Chaabouni Henda
Congiu Emanuele
Dulieu Francois
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