2024A&A...686A.139C

We present a study of CH₃CH₂CCH, CH₃CH₂CN, CH₂CHCCH, and CH₂CHCN in TMC-1 using the QUIJOTE line survey. We confirm the presence of CH₃CH₂CCH in TMC-1, which was previously reported as tentative by our group. From a detailed study of the ethynyl and cyanide derivatives of CH₂CH₂ and CH₃CH₃ in TMC-1, we found that the CH₂CHCCH/CH₂CHCN and CH₃CH₂CCH/CH₃CH₂CN abundance ratios are 1.5±0.1 and 4.8±0.5, respectively. The derived CH₂CHCCH/CH₃CH₂CCH abundance ratio is 15.3±0.8, and that of CH₂CHCN over CH₃CH₂CN is 48±5. All the single substituted isotopologs of vinyl cyanide have been detected, and we found that the first and second carbon substitutions in CH₂CHCN provide a ¹²C/¹³C ratio in line with that found for other three-carbon bearing species such as HCCNC and HNCCC. However, the third ¹³C isotopolog, CH₂CH¹³CN, presents an increase in its abundance similar to that found for HCCCN. Finally, we observed eight b-type transitions of CH₂CHCN, and we find that their intensity cannot be fitted adopting the dipole moment µ_b derived previously. These transitions involve the same rotational levels as those of the a-type transitions. From their intensity, we obtain µ_b = 0.80±0.03 D, which is found to be in between earlier values derived in the laboratory using intensity measurements or the Stark effect. Our chemical model indicates that the abundances of CH₃CH₂ CCH, CH₃CH₂CN, CH₂CHCCH, and CH₂CHCN observed in TMC-1 can be explained in terms of gas-phase reactions.