Reactivity Trends in the Gas-Phase Addition of Acetylene to the N-Protonated Aryl Radical Cations of Pyridine, Aniline, and Benzonitrile

Abstract

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion−molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta, and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10 −12 cm 3 molecule −1 s −1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10 −12 cm 3 molecule −1 s −1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10 −11 cm 3 molecule −1 s −1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height.

Modelling reaction kinetics of distonic radical ions: a systematic investigation of phenyl-type radical addition to unsaturated hydrocarbons

“…Potential energy schemes for the full reaction surface for the nine previously reported distonic ion–molecule reactions with acetylene can be found in that publication. 11…”

Spin‐Spin Coupling Controls the Gas‐Phase Reactivity of Aromatic σ‐Type Triradicals

“…The reactivity of the unknown biradical cation generated from 6 upon H atom abstraction resembled that of the authentic biradical cations 21 and 22 but was clearly different from that of 18. The relatively low abundance of background water adduct (Figure S1a, ion of m/z 146) suggests that the H-atomabstraction product of 6 is the 4,8-didehydroquinolinium cation (21) rather than the 5,8-didehydroquinolinium cation (22), which implies that the most reactive radical site in 6 is that at the C-5 position.…”

“…[7][8][9][10][11] Due to the difficulty of cleanly generating many mono-and biradicals in solution (with the exception of ortho-benzyne analogs), [1,12] the ion-molecule reactions of related charged mono-and biradicals have been studied in the gas phase by using mass spectrometry. [13][14][15][16][17][18][19][20][21][22][23] Based on the literature, radical reactions should be fairly insensitive to solvent effects, [24] which facilitates the comparison of the gas-phase and solution reactions. Furthermore, the gas-phase reactivity of several charged phenyl radicals and some charged aromatic biradicals has been demonstrated previously to be similar to the liquidphase reactivity of neutral phenyl radicals and aromatic biradicals with electron-withdrawing substituents, [25a-c] in spite of a recent demonstration that a charged substituent influences radical reactivity via through-space interactions while a neutral substituent influences it via through-bond interactions.…”

Atmospheric oxidation mechanism and kinetics of indole initiated by ●OH and ●Cl: a computational study

“…Detailed transformation pathways of a series of ONCs, including lowmolecular-weight alkyl amines (Nicovich et al, 2015;Xie et al, 2014Xie et al, , 2015F. F. Ma et al, 2021b), aromatic aniline (Xie et al, 2017;Shiels et al, 2021), heterocyclic amines (Sen-Gupta et al, 2010;Ma et al, 2018a;Borduas et al, 2016b;Ren and Da Silva, 2019) and amides (Xie et al, 2017;Borduas et al, 2016aBorduas et al, , 2015Bunkan et al, 2016Bunkan et al, , 2015, have been investigated. These studies have shown that the functional groups connected to the NH x (x = 0, 1, 2) group highly affect the reactivity of ONCs and eventually lead to their different atmospheric impacts.…”