Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions

Abstract

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical's dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C-H fission channel were also observed in the photolysis of propargyl chloride.

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

“…Theoretical investigations based on the quantum mechanical calculation of the minimum energy pathway suggest that the formation of non-cyclic final products generally dominates for energetic and/or entropic reasons, aided by rapid isomerization of any cyclic reaction intermediate to the more stable acyclic isomer. 73 However, these theoretical predictions are in disagreement with experimental studies on the CH + C 2 H 2 reaction by Boullart et al 76 and the unimolecular dissociation of the propargyl radical (C 3 H 3 ) by McCunn et al 77 both reporting cyclopropenylidene (c-C 3 H 2 ) as the main product. In order to address these discrepancies and to systematically determine the reaction mechanism(s) of CH radicals with small unsaturated linear hydrocarbons, the CH + C 2 H 4 , C 3 H 6 , C 3 H 4 , and C 2 H 2 systems have been studied using isomer-resolved photoionization mass spectroscopy, 44,65 CMB, [27][28][29] and computational calculations.…”

Theoretical insights into the reaction mechanisms between 2,3,7,8-tetrachlorodibenzofuran and the methylidyne radical

“…Its extreme reactivity arises from the presence of one singly occupied and one vacant nonbonding molecular orbital localized on the carbon atom. 37 Therefore, CH radical can react with most species ranging from small alkanes, [28][29][30][31] alkene, 28,[32][33][34][35][36][37][38] alkyne, 28,33,[37][38][39] pyrrole, 41 acrolein 42 to large polycyclic aromatic hydrocarbons like anthracene (C 14 H 10 ), 43 providing an alternative way to synthesize long-chain or ring-expanded hydrocarbons and complex organic molecules. For example, CH radical can react with pyrrole to produce the ring expansion product pyridine, 41 where CH radical can be readily prepared experimentally via the photolysis of bromoform at room temperature.…”

Slow electron velocity-map imaging spectroscopy of the 1-propynyl radical

“…1 The most stable of these, the propargyl radical ͑HC w C-CH 2 ͒, has been well studied and characterized in detail. [2][3][4][5][6][7][8][9] It has been determined to be an important intermediate in interstellar chemistry, 10,11 hydrocarbon combustion, 12,13 and hydrocarbon photochemistry. [14][15][16] The other isomers have been the subject of several theoretical studies, [17][18][19][20][21][22] but owing to the fact that they all lie 30-40 kcal/ mol higher in energy than propargyl, there is very little experimental information available on them.…”