Using doubly ionized acetylene as a superelectrophilic reagent, the new rare-gas compounds HCCAr2+ and HCCKr2+ have been prepared for the first time in hyperthermal collisions of mass-selected C2H2(2+) with neutral rare gases (Rg). However, electron transfer from the rare gas to the acetylene dication as well as proton transfer from C2H2(2+) to the rare gas efficiently compete with formation of HCCRg2+. The computational investigations show that the formation of HCCRg2+ from acetylene dication is endothermic with Rg = He, Ne, Ar and Kr and only weakly exothermic with Xe. These energetic factors, as well as the pronounced competition with the other reactive channels help to explain why HCCRg2+ is only observed with Rg = Ar and Kr.

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The products formed in the reaction between the methylidene radical (CH) and acrolein (CH2═CHCHO) are probed at 4 Torr and 298 K employing tunable vacuum-ultraviolet synchrotron light and multiplexed photoionization mass-spectrometry. The data suggest a principal exit channel of H loss from the adduct to yield C4H4O, accounting for (78 ± 10)% of the products. Examination of the photoionization spectra measured upon reaction of both CH and CD with acrolein reveals that the isomeric composition of the C4H4O product is (60 ± 12)% 1,3-butadienal and (17 ± 10)% furan. The remaining 23% of the possible C4H4O products cannot be accurately distinguished without more reliable photoionization spectra of the possible product isomers but most likely involves oxygenated butyne species. In addition, C2H2O and C3H4 are detected, which account for (14 ± 10)% and (8 +10, -8)% of the products, respectively. The C2H2O photoionization spectrum matches that of ketene and the C3H4 signal is composed of (24 ± 14)% allene and (76 ± 22)% propyne, with an upper limit of 8% placed on the cyclopropene contribution. The reactive potential energy surface is also investigated computationally, and specific rate coefficients are calculated with RRKM theory. These calculations predict overall branching fractions for 1,3-butadienal and furan of 27% and 12%, respectively, in agreement with the experimental results. In contrast, the calculations predict a prominent CO + 2-methylvinyl product channel that is at most a minor channel according to the experimental results. Studies with the CD radical strongly suggest that the title reaction proceeds predominantly via cycloaddition of the radical onto the C═O bond of acrolein, with cycloaddition to the C═C bond being the second most probable reactive mechanism.

Thermal reactions of the CF3 2+ dication with argon lead to the formation of an ArCF2 2+ dication, a new type of metastable species with an argon−carbon bond. None of the other rare gases undergo a similar reaction with CF3 2+. For the lighter rare gases (He and Ne), no reactions with CF3 2+ other than those due to electronically excited reactant ions are observed, whereas for the heavier rare gases (Kr and Xe), the prevailing reactive pathways involve single-electron transfer. At elevated collision energies, single-electron transfer predominates for collisions with all rare gases (He−Xe).

A B S T R A C TProducts formed in the reaction of C 2 H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C 6 H 6 . The photoionization spectrum of the C 6 H 6 product indicates that fulvene is formed with a branching fraction of (57 AE 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.Published by Elsevier B.V.