Discovery of the propargyl radical (CH₂CCH) in TMC-1: One of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds

Abstract

We present the first identification in interstellar space of the propargyl radical (CH2CCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40 m telescope. The six strongest hyperfine components of the 20, 2–10, 1 rotational transition, lying at 37.46 GHz, were detected with signal-to-noise ratios from 4.6σ to 12.3σ. We derived a column density of 8.7 × 1013 cm−2 for CH2CCH, which translates to a fractional abundance relative to H2 of 8.7 × 10−9. This radical has a similar abundance as methyl acetylene, with an abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thus one of the most abundant radicals detected in TMC-1, and it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud. We constructed a gas-phase chemical model and find calculated abundances that agree with, or fall two orders of magnitude below, the observed value depending on the poorly constrained low-temperature reactivity of CH2CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C2H4 reaction and by the dissociative recombination of C3$ \rm{H}_n^+ $ ions with n = 4−6. The propargyl radical is believed to control the synthesis of the first aromatic ring in combustion processes, and it probably plays a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds.

Interstellar hide and go seek: C3H4O. There and back (again)

“…Moreover, the proposed radiative stabilization of each isomer is consistent with the observation of the propargyl radical in the cold molecular cloud TMC-1. 58…”

“…Moreover, the proposed radiative stabilization of each isomer is consistent with the observation of the propargyl radical in the cold molecular cloud TMC-1. 58 We reiterate, while branching ratios in Fig. 6 provide insight into the unimolecular decay of precursor molecules (and the subsequent build up of intermediates) up to 100 ps, only dynamics studies can clarify lifetimes of further reactivity.…”

“…Although propargyl alcohol and allenol, which differ by OH coordination on either the CH 2 (head) or the CH (tail), are calculated to lie well above (4100 kJ mol À1 ) transacrolein, 1 the chemistry of the propargyl radical reveals many interesting connectivities: of particular note are the synthesis of polycyclic aromatic hydrocarbons (PAHs), and cyclization of benzene in cold dark clouds. 58,62,63 Statistical calculations suggest radical routes proceeding to both propargyl alcohol and allenol may exist, and are likely barrierless. 36,64 However, mechanisms to formation have not been rigorously examined.…”

“…Following the very recent interstellar discovery of the propargyl radical (H 2 CCCH˙) in TMC-1, 58 two additional C 3 H 4 O isomers have attracted our attention as prime candidates for observation: propargyl alcohol (HC 2 CH 2 OH) and allenol (H 2 CCCHOH), Fig. 2.…”

Organic Molecules in Interstellar Space: Latest Advances

“…The propargyl radical, CH 2 CCH, has recently been found in TMC-1 by Agúndez et al (2021b) with an abundance close to 10 -8 relative to H 2 . In addition, complex hydrocarbons such as vinyl and allenyl acetylene have also been observed with fairly large abundances (Cernicharo et al, 2021e;Cernicharo et al, 2021f).…”