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
  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 40m telescope. The six strongest hyperfine components of the
2,0,2-1,0,1 rotational transition, lying at 37.46~GHz, were detected with
signal-to-noise ratios in the range 4.6-12.3 sigma. We derive a column density
of 8.7e13 cm-2 for CH2CCH, which translates to a fractional abundance relative
to H2 of 8.7e-9. This radical has a similar abundance to 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 C3Hn+ 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.
