Reactivity Trends in the Gas-Phase Addition of Acetylene to the <i>N</i> -Protonated Aryl Radical Cations of Pyridine, Aniline, and Benzonitrile
Oisin J. Shiels, P. D. Kelly, Cameron C. Bright, Berwyck L. J. Poad, Stephen J. Blanksby, Gabriel da Silva, Adam J. Trevitt
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 cm3 molecule–1 s–1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10–12 cm3 molecule–1 s–1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10–11 cm3 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.