Ion-molecule reactivity monitoring with synchrotron radiation: reactions of CH2CN+ isomers with hydrocarbons
The atmosphere of Titan, Saturn’s largest satellite, hosts one of the most complex organic chemistry in the Solar system, initiated by N2 and CH4 and leading to the synthesis of complex hydrocarbons, nitriles and prebiotic molecules. Titan’s atmosphere is very similar to the Earth’s primordial atmosphere, thus understanding Titan atmospheric chemistry is extremely relevant for the chemical evolution of our planet.
Titan has a significant ionosphere and results from the Cassini-Huygens mission have demonstrated a strong implication of ionospheric chemistry in the synthesis of complex N-containing molecules, that maybe the precursors of stratospheric tholins.
Among N-containing ions, C2H2N+ have been detected on Titan, and during the STSM we have studied the reactivity of C2H2N+ isomers with CH4, C2H2 and C2H6. Using dissociative photoionization of appropriate neutral precursors, we have successfully demonstrated the possibility to generate different C2H2N+ isomers, namely the cyclic one (from the CH3CN precursor) and the cyanomethyl CH2CN+ cation (from the ICH2CN precursor). The experiments have been performed using the CERISES set-up, a guided ion beam mass spectrometer that permits the measurement of absolute reactive cross sections and branching ratios as a function of photon and collision energies.
Among the most relevant results we mention:
a) in the case of CH4, the CH2CN+ cation is the only reactive isomer, and one of the three most abundant product is C3H4N+ (plus H2) in which a new C-C bond has been formed
b) in the case of C2H2, both c-C2H2N+ and CH2CN+ are responsible for the synthesis of the most abundant C3H3+ product, while minor channels CH3+ (plus HC3N) and C4H2N+ (plus H2) derive exclusively from the linear isomer, as shown in the Figure.
The experiments have been carried out in a joint collaboration among the Host Institution in Orsay/SOLEIL Synchrotron (C. Alcaraz, C. Romanzin, R. Thissen), the Trento group (D. Ascenzi), the Stockholm group (W. Geppert) and the Prague team (M. Polasek) and we are confident that they will results in at least one scientific publication.