A combined crossed molecular beams and computational study on the formation of distinct resonantly stabilized C5H3 radicals via chemically activated C5H4 and C6H6 intermediates

Abstract

The crossed molecular beams technique was utilized to explore the formation of three isomers of resonantly stabilized (C₅H₃) radicals along with their d₂-substituted counterparts via the bimolecular reactions of singlet/triplet dicarbon [C₂(X¹Σ⁺_g/a³Π_u)] with methylacetylene [CH₃CCH(X¹A₁)], d₃-methylacetylene [CD₃CCH(X¹A₁)], and 1-butyne [C₂H₅CCH(X¹A′)] at collision energies up to 26 kJ mol⁻¹via chemically activated singlet/triplet C₅H₄/C₅D₃H and C₆H₆ intermediates. These studies exploit a newly developed supersonic dicarbon [C₂(X¹Σ⁺_g/a³Π_u)] beam generated via photolysis of tetrachloroethylene [C₂Cl₄(X¹A_g)] by excluding interference from carbon atoms, which represent the dominating (interfering) species in ablation-based dicarbon sources. We evaluated the performance of the dicarbon [C₂(X¹Σ⁺_g/a³Π_u)] beam in reactions with methylacetylene [CH₃CCH(X¹A₁)] and d₃-methylacetylene [CD₃CCH(X¹A₁)]; the investigations demonstrate that the reaction dynamics match previous studies in our laboratory utilizing ablation-based dicarbon sources involving the synthesis of 1,4-pentadiynyl-3 [HCCCHCCH(X²B₁)] and 2,4-pentadiynyl-1 [H₂CCCCCH(X²B₁)] radicals via hydrogen (deuterium) atom elimination. Considering the C₂(X¹Σ⁺_g/a³Π_u)–1-butyne [C₂H₅CCH(X¹A′)] reaction, the hitherto elusive methyl-loss pathway was detected. This channel forms the previously unknown resonantly stabilized penta-1-yn-3,4-dienyl-1 [H₂CCCHCC(X²A)] radical along with the methyl radical [CH₃(X²A₂′′)] and is open exclusively on the triplet surface with an overall reaction energy of −86 ± 10 kJ mol⁻¹. The preferred reaction pathways proceed first by barrierless addition of triplet dicarbon to the π-electronic system of 1-butyne, either to both acetylenic carbon atoms or to the sterically more accessible carbon atom, to form the methyl-bearing triplet C₆H₆ intermediates [i4_1b] and [i8_1b], respectively, with the latter decomposing via a tight exit transition state to penta-1-yn-3,4-dienyl-1 [(H₂CCCHCC(X²A)] plus the methyl radical [CH₃(X²A₂′′)]. The successful unraveling of this methyl-loss channel – through collaborative experimental and computational efforts – underscores the viability of the photolytically generated dicarbon beam as an unprecedented tool to access reaction dynamics underlying the formation of resonantly stabilized free radicals (RSFR) that are vital to molecular mass growth processes that ultimately lead to polycyclic aromatic hydrocarbons (PAHs).