A study of C₄H₃N isomers in TMC-1: Line by line detection of HCCCH₂CN

Abstract

We present Yebes 40 m telescope observations of the three most stable C4H3N isomers towards the cyanopolyyne peak of TMC-1. We have detected 13 transitions from CH3C3N (A and E species), 16 lines from CH2CCHCN, and 27 lines (a-type and b-type) from HCCCH2CN. We thus provide a robust confirmation of the detection of HCCCH2CN and CH2CCHCN in space. We have constructed rotational diagrams for the three species and obtained rotational temperatures between 4 and 8 K as well as similar column densities for the three isomers, in the range (1.5−3) × 1012 cm−2. Our chemical model provides abundances of the order of the observed ones, although it overestimates the abundance of CH3CCCN and underestimates that of HCCCH2CN. The similarity of the observed abundances of the three isomers suggests a common origin, most probably involving reactions of the radical CN with the unsaturated hydrocarbons methyl acetylene and allene. Studies of reaction kinetics at low temperatures and further observations of these molecules in different astronomical sources are needed to draw a clear picture of the chemistry of C4H3N isomers in space.

Detection of deuterated methylcyanoacetylene, CH2DC3N, in TMC-1

“…1, the agreement between the synthetic spectrum and the observations is excellent. The column density is not very sensitive to the adopted value of the rotational temperature between 6 and 10 K. For the normal isotopologue Marcelino et al (2021) derived a rotational temperature for the A and E species of 6.7±0.2 K and of 8.2±0.6 K, respectively. They derived a total column density for CH 3 C 3 N of (1.74±0.1)×10 12 cm −2 .…”

“…Even though the equilibrium rotational constants differ slightly from the ground state constants, we can assume similar discrepancies for CH 3 C 3 N and CH 2 DC 3 N, and thus that the estimated constants for CH 2 DC 3 N are essentially unaffected. Methyldiacetylene (CH 3 C 4 H) is 7.5 times more abundant than, and lines have similar intensities to, CH 3 C 3 N (Marcelino et al 2021;Cernicharo et al 2021c). Hence, it is straightforward to think that spectral signatures of the deuterated species of CH 3 C 4 H could also be detected in our line survey.…”

“…We only consider gas-phase mechanisms that allow quantitative predictions based on some experimental measurements and theoretical studies. The chemistry of the different C 4 H 3 N and C 5 H 4 isomers has been discussed recently in Cernicharo et al (2021c) and Marcelino et al (2021), respectively, in relation with their detection in TMC-1. These two chemical families are tightly linked to the chemistry of methylacetylene, CH 3 CCH, and its isomer allene , CH 2 CCH 2 : Considering the deuterated analogues of these reactions introduces diverse questions, for example whether the CN reactions proceed without changing the methyl radical or lead to some scrambling of the hydrogen atoms in a quasi-stationary intermediate followed by different reaction channels.…”

Discovery of the acetyl cation, CH3CO+, in space and in the laboratory

“…3.3). The symmetric top CH 3 C 3 N is discussed in Marcelino et al (2021). Figure D.2 shows the J = 2 − 1 transition for CH 3 CN, CH 3 CCH, CH 3 NC, and CH 3 CO + (for this species, see also Fig.…”

Detection of interstellar H2CCCHC3N

“…This species, recently analyzed in the laboratory by McCarthy et al (2020), could plausibly represent such a chemical "missing link". The first detection of the unsubstituted form of this molecule, allenyl acetylene (H 2 CCCHCCH), also in TMC-1, has recently been reported by Cernicharo et al (2021b); they also detected a number of new interstellar species in that source using data taken with the Yebes 40 m radio telescope (Cernicharo et al 2021a,c;Marcelino et al 2021;Cabezas et al 2021;Agundez et al 2021), thereby allowing for further examinations of their abundances and potential chemical connections.…”