STSM by Andrea Cernuto, University of Trento (IT)  with Christian Alcaraz, Université Paris-Sud, Orsay (FR)
On May 21st, 2014 (7 days)

Effect of internal and kinetic energy on the reactions of methyl cations with hydrocarbons

The purpose of the STSM to the Soleil Synchrotron (Paris) was to study the reactivity of CH3+ with a varying degree of internal excitation with CH4, allene (C3H4) and 2-butyne (C4H6). These reactions are of relevance for the modelling of high energy environments such as planetary atmospheres and plasma systems. In fact the methyl carbocation has been detected in the atmosphere of Titan and in laboratory plasmas, e.g. those used for the conversion of methane (in some cases mixed with CO2) into higher hydrocarbons.
The experiments have been performed, at the DESIRS beamline of the Soleil Synchrotron, with a multipole QOOQ mass spectrometer set-up, that is optimized to study ion-molecule reactions as a function of the collision energy. The CH3+ cation is produced by VUV photoionization of CH3 radicals generated in a molecular beam seeded in Ar and coupled to a flash pyrolysis source using CH3NO2 as precursor. The internal excitation of the parent cation can be varied by changing the photon energy from the threshold ionization of CH3 (about 9.8 eV) to 12.5 eV.
For the reaction of CH3+ with methane we expected two products, C2H3+ (m/z 27) and C2H5+ (m/z 29): the results about this system clearly show the change in the branching ratios of the two products at different photon energies and a huge effect of the internal energy on the cation reactivity. The last system studied is the reaction between CH3+ and 2-butyne. The reactivity can be summarized by the following energy scheme.


Fig. 1. Guess of energy values referred to NIST.

The main reactions are the charge transfer (C4H6+), the extraction of H- (C4H5+) and the condensation (C5H7+) path. Very interesting to study are the channels leading to the formation of C3H3+ and C2H3+: in fact, these clearly show the effect of the internal excitation of CH3+, because they are either endothermic or thermoneutral processes. In addition, we can confirm that the other pathways are exothermic and in general insensitive to the effect of the internal excitation of the primary ion.

The obtained results are encouraging because they show the effect of the internal excitation on the reaction of CH3+ with unsatured hydrocarbons.The results hereby obtained are a successful proof of principle demonstrating the feasibility of studying state-selected reactivity of CH3+ cations using the photon energy.
Future collaboration with the host institution can be foreseen in the form of further synchrotron campaigns to complete and quantify the study initiated. The experimental results will need to be carefully analyzed but we can predict that at least one publication (in the form of Communication or Letter) will result from the STSM.


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