Mass Spectrometric and Computational Studies on the Reaction of Aromatic Peroxyl Radicals with Phenylacetylene Using the Distonic Radical Ion Approach

Product and mechanistic studies were performed for the reaction of aromatic distonic peroxyl radical cations 4-PyrOO^•+ and 3-PyrOO^•+ with phenylacetylene (7) in the gas phase using mass spectrometric and computational techniques. PyrOO^•+ was generated through reaction of the respective distonic aryl radical cation Pyr^•+ with O₂ in the ion source of the mass spectrometer. For the reaction involving the more electrophilic 4-PyrOO^•+, a rate coefficient of k₁ = (2.2 ± 0.6) × 10^–10 cm³ molecule^–1 s^–1 was determined at 298 K, while a value of k₂ = (8.2 ± 2.1) × 10^–11 cm³ molecule^–1 s^–1 was obtained for the reaction involving the less electrophilic 3-PyrOO^•+. This highlights the role of polar effects in these reactions, which are likely of high relevance for processes in combustions and atmospheric transformations. The mechanism was studied by computational methods, which showed that radical addition occurs exclusively at the less substituted alkyne site to give the distonic vinyl radical cation 8. The latter undergoes a series of subsequent rearrangements/fragmentations that are similar for both isomeric PyrOO^•+. γ-Fragmentation in 8 leads to the distonic aryloxyl radical cation PyrO^•+ and a singlet carbene 10. The product association complex [PyrO^•+ – 10] is the starting point for two important subsequent reactions, e.g., (i) rapid hydrogen transfer to form ketenyl radical 11 and the closed-shell species PyrOH⁺, and (ii) oxygen transfer from PyrO^•+ to 10 that leads to α-keto aldehyde 13 and Pyr^•+, followed by hydrogen abstraction to give acyl radical 14 and PyrH⁺. Additional major products are the closed-shell aromatic carbonyl compounds 20 and 30 that result from multistep rearrangements in vinyl radical 8, which are terminated by homolytic bond scission and release of neutral acyl radicals.