Barrierless Reactions of Three Benzonitrile Radical Cations with Ethylene
© 2020 CSIRO. Reactions of three protonated benzonitrile radical cations with ethylene are investigated. Product branching ratios and reaction kinetics, measured using ion-trap mass spectrometry, are reported and mechanisms are developed with support from quantum chemical calculations. Reactions proceed via pre-reactive van der Waals complexes with no energy barrier (above the reactant energy) and form radical addition and addition-elimination product ions. Rate coefficients are 4-dehydrobenzonitrilium: 1.72 ± 0.01 × 10-11 cm3 molecule-1 s-1, 3-dehydrobenzonitrilium: 1.85 ± 0.01 × 10-11 cm3 molecule-1 s-1, and 2-dehydrobenzonitrilium: 5.96 ± 0.06 × 10-11 cm3 molecule-1 s-1 (with ±50 % absolute uncertainty). A ring-closure mechanism involving the protonated nitrile substituent is proposed for the 2-dehydrobenzonitrilium case and suggests favourable formation of the protonated indenimine cation.