Discovery of the propargyl radical (CH

We present the first identification in interstellar space of the propargyl radical (CH(2)CCH). 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 σ. We derive a column density of 8.7 × 10(13) cm(−2) for CH(2)CCH, which translates to a fractional abundance relative to H(2) of 8.7 × 10(−9). This radical has a similar abundance to methyl acetylene, with an abundance ratio CH(2)CCH/CH(3)CCH 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 CH(2)CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C(2)H(4) reaction and by the dissociative recombination of C(3)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.