Molecular Weight Growth in the Gas-Phase Reactions of Dehydroanilinium Radical Cations with Propene.

Abstract

Protonated aniline-one of the simplest nitrogen-bearing aromatic molecules-is speculated to be present within Titan's atmosphere, where it could play a role in molecular weight growth chemistry. To investigate this possibility, this paper examines the reactions of propene with distonic radical cations derived from protonated aniline. The reaction kinetics, products, and branching ratios of these distonic radical cations (i.e., 2-, 3-, and 4-dehydroanilinium radical cations) are measured in the gas phase using ion-trap mass spectrometry, and calculations (M06-2X/6-31G(2df,p) and G3X-K) are used to rationalize the results. The reaction efficiencies for the 2-, 3-, and 4-dehydroanilinium radical cations are reported as 30.7 (±3.1)%, 2.4 (±0.5)%, and 2.1 (±0.4)%, respectively (with ±50% absolute accuracy). The 4- and 3-dehydroanilinium radical cations undergo addition of propene followed by either (i) methyl radical loss forming ammonio styrene cations (m/z 120) or (ii) ethylene loss pathways forming the ammonio benzyl radical cation (m/z 107). The 4-dehydroanilinium reaction favors methyl loss over ethylene loss, whereas the reverse is true for 3-dehydroanilinium. In contrast, the 2-dehydroanilinium radical cation reacts with propene to purportedly form amino tropylium cation (m/z 106) with ethyl radical loss and methyl amino tropylium cation (m/z 120) with methyl radical loss. The energies of all minima and transition states leading to predicted dissociation products are below the energy of the initial reactants providing plausible pathways at the low temperatures of Titan's atmosphere.