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OATS-UNI/TS SEMINAR
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Speaker: Nadia Balucani (Univ. Perugia)

Title: The reactions of atomic oxygen with unsaturated hydrocarbons in extraterrestrial environments
Date: Wednesday, January 16th, 2013
Time: 11.30
Venue: Villa Bazzoni
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Abstract: The observation in hydrocarbon-rich planetary atmospheres and interstellar clouds of highly unsaturated organic molecules (such as acetylene) poses the question of how these molecules can survive in the upper atmospheres where atomic and radical species are relatively abundant. From a chemical point of view, indeed, each unsaturated C-C bond is a potential site for addition reactions especially when radicals are involved. Part of this puzzle is actually explained by the fact that the reactions of small alkynes, alkenes and dienes with several atomic and radical species (such as atomic carbon or CH, C2H, C2 and CN radicals) do not destroy the C-atom skeleton of unsaturated species, but rather elongate it. Notable examples are the reactions between C, C2, C2H, CH, CN with acetylene (HCCH) and diacetylene (HCCCCH) which, in laboratory experiments, have been verified to generate the products C3, C3H, C3H2, C4H, C5H, C6H, C6H2, HC3N and HC5N. Some of these species have been identified in hydrocarbon-rich atmospheres, including the atmosphere of Titan or several interstellar objects. Quite interestingly, analogous oxygen-containing highly unsaturated species have not been identified and oxygen is mostly locked in CO2, H2CO and CO. In our laboratory we have started a systematic investigation of several reactions involving atomic oxygen and unsaturated hydrocarbons (C2H2, C2H4, CH3CCH, CH2=C=CH2) by means of the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis. Quite interestingly, we have observed that C-C bond fission channels are always relevant or dominant reaction pathways. The most illustrative case is the reaction between O and allene (CH2=C=CH2) where ~ 90% of the reaction leads directly to the stable products CO+C2H4, but also significant fractions of the reactions O+C2H2and O+C2H4 lead to the products CO+CH2 and HCO+CH3, respectively. In other words, the reactions of unsaturated hydrocarbons with atomic oxygen are not only terminating the hydrocarbon growth in oxygen-rich environments, but are also able to convert relatively abundant and widely spread small unsaturated hydrocarbons directly into CO or its precursor HCO. Implications of the carbon-to-oxygen number density ratio on the chemistry of planetary atmospheres and interstellar medium will be noted.
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Contact: Giovanni Vladilo (OATs)