DOE OSTI2018
The crossed molecular beams technique was utilized to investigate the formation of three isomers of resonantly stabilized (C 5 H 3 ) radicals along with their d 2 -substituted counterparts via the bimolecular reactions of singlet/triplet dicarbon [C 2 (X 1 Σ + g /a 3 Π u )] with methylacetylene [CH 3 CCH(X 1 A 1 )], d 3 -methylacetylene [CD 3 CCH(X 1 A 1 )], and 1-butyne [C 2 H 5 CCH(X 1 A')] at collision energies up to 26 kJ mol -1 via chemically activated singlet/triplet C 5 H 4 /C 5 D 3 H and C 6 H 6 intermediates. These studies exploit a newly developed supersonic dicarbon [C 2 (X 1 Σ + g /a 3 Π u )] beam generated via photolysis of tetrachloroethylene [C 2 Cl 4 (X 1 A g )] by excluding interference from carbon atoms, which represent the dominating (interfering) species in ablation-based dicarbon sources. We assessed the performance of the dicarbon [C 2 (X 1 Σ + g /a 3 Π u )] beam in reactions with methylacetylene [CH 3 CCH(X 1 A 1 )] and d 3 -methylacetylene [CD 3 CCH(X 1 A 1 )]; 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(X 2 B 1 )] and 2,4-pentadiynyl-1 [H 2 CCCCCH(X 2 B 1 )] radicals via hydrogen (deuterium) atom elimination. Considering the C 2 (X 1 Σ + g /a 3 Π u )–1-butyne [C 2 H 5 CCH(X 1 A')] reaction, the hitherto elusive methyl-loss pathway was detected. This channel forms the previously unknown resonantly stabilized penta-1-yn-3,4-dienyl-1 [H 2 CCCHCC(X 2 A)] radical along with the methyl radical [CH 3 (X 2 A 2 ")] 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 C 6 H 6 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 [(H 2 CCCHCC(X 2 A)] plus the methyl radical [CH 3 (X 2 A 2 ")]. 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)
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗