Five-membered ring compounds from the ortho-benzyne + methyl radical reaction under interstellar conditions

Abstract

Reactive open-shell species, such as radicals and biradicals, are key intermediates in the formation of (poly)cyclic hydrocarbon species in a variety of interstellar environments, ranging from cold molecular clouds to the outflows of carbon-rich stars. In this work, we identify the products of the o-benzyne + methyl radical reaction isomer-selectively by photoion mass-selected threshold photoelectron spectroscopy. We assign the benzyl (C 7 H • 7 ) radical as the sole intermediate of the association reaction. Subsequent hydrogen atom loss from benzyl yields the five-membered ring species fulvenallene (FA), 1-ethynylcyclopentadiene (1ECP), and 2-ethynylcyclopentadiene (2ECP) which have recently been detected in the cold molecular cloud TMC-1. We report a comprehensive C 7 H 7 potential energy surface (PES) of the title reaction and show that the products form via direct barrierless addition followed by ring-contraction and hydrogen elimination. A statistical model predicts 89% 1ECP, 8% FA, and 3% 2ECP branching ratios at 0 K. Astrochemical simulations of TMC-1 incorporating this reaction result in the excellent reproduction of the abundance of a five-membered ring species, 1ECP, and provide strong evidence for the in situ "bottomup" formation of small cyclic species in cold cores. Last, we put the results in context of the recent detection of fulvenallene in TMC-1.

The Identity and Chemistry of C7H7 Radicals Observed during Soot Formation

“…(a) PI spectra are shown for m / z 91 in the methane flame at three HABs compared with the average PI spectrum for the three HABs in the butane flame (see legend). (b) PI spectra from the flame are compared with reference curves for benzyl, , o -tolyl, tropyl, ,,, and vinylcyclopentadienyl, , as described in the Supporting Information. The solid circles in panel b are the same data as shown in panel a, and the solid line without markers represents the fit to the data.…”

“…Section D of the Supporting Information describes the method for scaling and merging the vinylcyclopentadienyl data. We used a PI spectrum for o-tolyl derived from the TPES calculated by Bouwman et al, 53 which is shown in Figure S10 and described in Section E of the Supporting Information. Section F of the Supporting Information describes the 3-ethynylcyclopentenyl reference curve used in this study (shown in Figure S11), which was reproduced from Jin et al 55 who derived the PI spectrum from an analysis of a TPES provided by Bierkandt et al 65 The merged reference curves used in this study are also provided in tabulated form in the Supporting Information.…”

“…54,71,72 Derudi et al 73 suggested an additional route for benzyl decomposition to benzyne + methyl, which is supported by the experimental results of Buckingham et al 64 The work of Marti ét al. 54 indicates that this pathway may be important at temperatures greater than 1000 K. Bouwman et al 53 identified benzyl production through the reverse reaction of o-benzyne + methyl during low-pressure pyrolysis.…”

“…Whereas benzyl is a significant contributor to the flame samples, especially the ethylene flame, it contributes little or not at all under the butane pyrolysis conditions. Tropyl, on the other hand, appears to be a significant component in pyrolysis but is less significant in the methane and butane flames and 46,95 o-tolyl, 53 tropyl, 46,55,59,94 and vinylcyclopentadienyl, 46,55 The uncertainties indicate the standard deviation of the fit. influenced by the addition of the C 7 H 7 reactions for either mechanism.…”

“…Radical species with the chemical formula C 7 H 7 are part of the sequence of RSRs observed during soot formation and are proposed to be important to soot inception . Aerosol mass spectra of particles produced in sooting flames and pyrolysis systems have demonstrated the production of species with a mass of 91 u, corresponding to one or more isomers of C 7 H 7 . ,, C 7 H 7 species have also been investigated as intermediates in the formation of larger PAHs in interstellar environments. , Martí et al recently performed an extensive theoretical study of the isomers of C 7 H 7 and calculated rate coefficients for C 7 H 7 production, loss, and isomerization reactions. Figure provides a summary of what appear to be the most important reactions, as suggested by the large breadth of experimental and theoretical work published previously.…”