Dissociative recombination of protonated nitriles with implications for the nitrile chemistry of Titan's upper atmosphere

Details Dissociative recombination of protonated nitriles with implications for the nitrile chemistry of Titan's upper atmosphere Dissociative recombination of protonated nitriles with implications for the nitrile chemistry of Titan's upper atmosphere

Apr 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010ttt..work...78v&link_type=abstract

Through Time; A Workshop On Titan's Past, Present and Future, NASA Goddard Space Flight Center, April 6th - 8th, 2010. Edited b

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Scientific paper

The Cassini mission has delivered many new insights about the atmosphere of Titan. It has been realized that the nitrogen chemistry and in particular the nitrile chemistry is far more evolved than previously thought. The ion spectrum from the T5 flyby of Cassini on the 16th of April 2005 indicated unexpectedly high abundances of protonated aceto-, acrylo- and propionitrile, which in turn points towards high concentrations of the neutral species. These nitriles can polymerize and diffuse downwards to contribute to the formation of tholins (which can hydrolyse to form biomolecule precursors as amino acids and nuclebases), but due to their high proton affinities they are on the other hand easily protonated by the ubiquitous hydrocarbon ions in Titan's atmosphere. Protonated nitriles can then undergo dissociative recombination, which involves the capture of a free electron to form a highly excited intermediate molecule, which rapidly dissociates into neutral fragments. This reaction sequence is one of the most important destruction mechanisms for nitriles in Titan's atmosphere. Unfortunately experimental data on the reaction rates and products of dissociative recombination of many protonated nitriles is still lacking. The dissociative recombination of protonated aceto-, acrylo- and propionitrile have been investigated at the heavy ion storage ring CRYRING, at the Manne Siegbahn Laboratory in Stockholm, Sweden. CRYRING enables the measurement of rate coefficients of such processes at the low collision energies prevalent in Titan's atmosphere. Thermal rate coefficients have been deduced for electron temperatures ranging from ~10 to 1000 K. The branching fractions of the reactions have also been determined and it has been shown that for all ions a significant fraction (43-65%) of the dissociative recombination events preserve the heavy atom chain. This indicates that neutral nitriles that are lost by protonation in Titan's upper atmosphere can be recycled by dissociative recombination to a large extent. Possible implications of these results on chemical models of Titan's upper atmosphere are discussed.

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