Abstract
Experimental and theoretical studies are presented on the reactivity of the radical cation isomers H2CNH+• (methanimine) and HCNH2+• (aminomethylene) with ethyne (C2H2). Selective isomer generation is performed via dissociative photoionization of suitable neutral precursors as well as via direct photoionization of methanimine. Reactive cross sections (in absolute scales) and product branching ratios are measured as a function of photon and collision energies. Differences between isomers’ reactivity are discussed in light of ab-initio calculations of reaction mechanisms. The major channels, for both isomers, are due to H atom elimination from covalently bound adducts to give [C3NH4]+. Theoretical calculations show that while for the reaction of HCNH2+• with acetylene any of the three lowest energy [C3NH4]+ isomers can form via barrierless and exothermic pathways, for the H2CNH+• reagent the only barrierless pathway is the one leading to the production of protonated vinyl cyanide (CH2CHCNH+), a prototypical branched nitrile species that has been proposed as a likely intermediate in star forming regions and in the atmosphere of Titan. The astrochemical implications of the results are briefly addressed.
Highlights
The reaction of ions is thought to play a crucial role in the synthesis of complex organic species in both the interstellar medium (ISM) and the atmospheres of planets and their satellites Larsson et al (2012)
The photoionization efficiency (PIE) curves for the parent radical cation H2CNH+ (m/z 29) and the photo-dissociation product HCNH+ (m/z 28) are reported in Figure 1, where the observed Appearance Energies (AEs) are given
For H2CNH+, an Ionization Energy (IE) of 10.01 ± 0.08 eV was obtained using the method of linear threshold extrapolation of the PIE curve at m/z 29 Chupka (1971), Traeger and McLoughlin (1981), Castrovilli et al (2014), a value that is fully consistent with the adiabatic ionization energy of methanimine of 9.99 eV as determined via mass-selected threshold photoelectron spectroscopy Holzmeier et al (2013)
Summary
The reaction of ions is thought to play a crucial role in the synthesis of complex organic species in both the interstellar medium (ISM) and the atmospheres of planets and their satellites Larsson et al (2012). The existence of isomeric ions in the ISM has been known for a long time and in some cases, such as HCO+ and HOC+, a determination has been made of their relative abundances Woods et al (1983). While isomers often have quite different dipole moments and other spectroscopic properties and have distinct radioastronomic profiles Woods et al (1983), in situ measurements of ion abundances in planetary and satellite atmospheres often employ mass spectrometers which bring extreme sensitivity and time/localisation specific information, but cannot distinguish between isomers. In order to develop accurate models to predict these abundances, it is important to have reliable data on the reactivity of ionic isomers with common interstellar and atmospheric molecules
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