Growth of Larger Hydrocarbons in the Ionosphere of Titan

Abstract

Among the many fascinating results of the Cassini-Huygens mission, the mass spectrum of the ionosphere of Titan has attracted considerable attention. [1] In brief, the ionosphere was found to be surprisingly complex, consisting of hydrocarbon ions C m H n + as well as nitrogen-containing ions C n H n N o + with mass-to-charge ratios up to the probes limit of m/z 100; [2] even much heavier components have been proposed. [1b, 3] While the formation of C m H n compounds with m 7 is reasonably well understood, [3][4][5] routes to larger hydrocarbons are less obvious. Moreover, most of the present models rely on condensation reactions of C m H n + ions with unsaturated precursors such as acetylene, [6] whereas methane, as the major hydrocarbon in the atmosphere of Titan, only plays a minor role in the subsequent growth processes. Here, we report carbon À carbon (C À C) coupling reactions of methane with medium-sized C m H n 2 + dications leading to larger hydrocarbon molecules. Despite low steady-state concentrations of the dicationic intermediates, kinetic modeling allows predictions about the larger hydrocarbon species present in the ionosphere of Titan, thereby rationalizing the results from the Cassini-Huygens mission which consideration of monocations only cannot explain.The activation of methane poses a particular challenge and usually involves energetic conditions or metal catalysis. [7] Under the conditions of the Titan atmosphere (low temperatures and pressures), small hydrocarbon ions can indeed react with methane, but the rate constants decrease with size, and so far reaction 1 involves the largest C m H n + ion reacting with methane under thermal conditions. [8, 9] C 6 H 5Recently, we proposed double ionization as a feasible route for C À C bond formation under extreme conditions. [10] In our laboratory experiments (see Supporting Information), C m H n + mono-and C m H n 2 + dications (m = 7-11, n = 6-12) were generated by electron ionization (EI) of aromatic precursors, mass-selected, and allowed to interact with methane.[11] Whereas most C m H n + monocations studied do not show a significant reactivity with methane under these conditions, many C m H n 2 + dications undergo dehydrogenative C À C coupling according to reaction (2); we note in passing that none of these C m H n 2 + dications reacts with nitrogen as the major component in the atmosphere of Titan.

Infrared spectroscopy of CHCl2+ molecular dications

“…Thus, fragmentation to another dication and a neutral fragment is usually energetically disfavored [6]. Nevertheless, for hydrocarbon dications, it was shown that bond-forming reactions where twofold charge is preserved can represent the dominant reactivity channel [12,13]. The trick involves opening a low energy exit channel denoted by elimination of an H atom of a H 2 molecule [14][15][16].…”

Formation of Organoxenon Dications in the Reactions of Xenon with Dications Derived from Toluene

“…[1,2] The precursor dications in these studies were generated by electron ionization, which is known as a "hard" ionization method. For reference purposes, in these previous studies we also investigated the unimolecular fragmentations of the metastable reactant dications, as well as their fragmentation in the presence of rare gases as, supposedly, chemically inert collision partners.…”

Activation of Methane by Gaseous Metal Ions

“…[10] Most of these reactions are driven by the radicaloid nature of the oxide cations with the spin mostly located on oxygen, which leads to a high preference for hydrogen-atom abstraction from methane to generate methyl radicals. This situation is in contrast to the dehydrogenation of methane by various 5d elements to give metal carbenes MCH 2 + , [6,11] or MCH 2 2+ , [6,12] and to the C À C coupling reactions with methane observed for medium-sized hydrocarbon dications, [13] in which the dehydrogenation takes place with the "CH 2 " fragment remaining at the ionic species.…”