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We present the first identification in interstellar space of the propargylradical (CH2CCH). This species was observed in the cold dark cloud TMC-1 usingthe Yebes 40m telescope. The six strongest hyperfine components of the2,0,2-1,0,1 rotational transition, lying at 37.46~GHz, were detected withsignal-to-noise ratios in the range 4.6-12.3 sigma. We derive a column densityof 8.7e13 cm-2 for CH2CCH, which translates to a fractional abundance relativeto H2 of 8.7e-9. This radical has a similar abundance to methyl acetylene, withan abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thusone of the most abundant radicals detected in TMC-1, and it is probably themost abundant organic radical with a certain chemical complexity ever found ina cold dark cloud. We constructed a gas-phase chemical model and findcalculated abundances that agree with, or fall two orders of magnitude below,the observed value depending on the poorly constrained low-temperaturereactivity of CH2CCH with neutral atoms. According to the chemical model, thepropargyl radical is essentially formed by the C + C2H4 reaction and by thedissociative recombination of C3Hn+ ions with n = 4-6. The propargyl radical isbelieved to control the synthesis of the first aromatic ring in combustionprocesses, and it probably plays a key role in the synthesis of large organicmolecules and cyclization processes to benzene in cold dark clouds.

astronomy and astrophysics

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