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

Abstract

Protonated aniline-one of the simplest nitrogen-bearing 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 and ion-trap mass spectrometry and calculations (M06-2X/6-31G(2df,p) and G3X-K) are used to characterize each reaction. 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 (i) methyl radical loss forming ammonio styrene cations (m/z 120) and (ii) ethylene loss pathways forming the ammonio benzyl radical cation (m/z 107). The 4-dehydroanilinium reaction favours the 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) by ethyl radical loss and methyl amino tropylium cation (m/z 120) by methyl radical ejection. 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 lower temperatures of Titan's atmosphere.

Formation of Protonated ortho-Quinonimide from ortho-Iodoaniline in the Gas Phase by a Molecular-Oxygen-Mediated, ortho-Isomer-Specific Fragmentation Mechanism

“…It appears that the 2-dehydroanilinium ion is so reactive that it undergoes complete annihilation in the ion source. The higher reactivity of the 2-dehydroanilinium ion compared to that of the 3- or 4-dehydroanilinium ion has been noted recently by Kelly et al Thus, the m / z 93 ion delivered from the ion source to the mobility cell is the nonreactive molecular radical-cation of aniline (Supporting Information Figure S6a).…”

“…Reactions between gaseous ions and neutral molecules have been investigated from the early stages of mass spectrometry. − Many examples have been documented of adventitious or deliberate ion–molecule reactions (IMR) that occur between residual neutral gas molecules and nontargeted or mass-selected ions. − Such ion–molecule reactions are often useful as probes for molecular structure and functional group determinations. − Some of these reactions have been particularly useful for the differentiation of isomeric radical ions. − Ion–molecule reactions are usually investigated using ion-storing mass analyzers such as ion traps and Fourier-transform ion cyclotron resonance instruments . Occasionally, the formation of ion–molecule reaction adducts have also been observed on tandem-in-space instruments as well. ,, …”

Reactivity Trends in the Gas Phase Addition of Acetylene to N-protonated Aryl Radical Cations

“…As previously reported, isomerisation between these nine radical cations is negligible under the experimental conditions due to large potential energy barriers for hydrogen atom migration. [29][30][31][32][33][34][35][36] For the Anl radicals however, appreciable population of unreactive ions at m/z 93 are also detected, especially in the 2Anl case. This unreactive ion population is attributed to formation of the conventional radical cation along with the target distonic radical ions (as noted in Ref [31]) and its origins will be discussed in further detail below.…”

“…Another minor peak at m/z 125 corresponds to background O2 addition to the radical cation, which has been reported previously. 30,36…”

Reactivity Trends in the Gas Phase Addition of Acetylene to N-protonated Aryl Radical Cations

“…As previously reported, isomerisation between these nine radical cations is negligible under the experimental conditions due to large potential energy barriers for hydrogen atom migration. [29][30][31][32][33][34][35][36] For the Anl radicals however, appreciable population of unreactive ions at m/z 93 are also detected, especially in the 2Anl case. This unreactive ion population is attributed to formation of the conventional radical cation along with the target distonic radical ions (as noted in Ref [31]) and its origins will be discussed in further detail below.…”