CN and CCH derivatives of ethylene and ethane: Confirmation of the detection of CH_3CH_2CCH in TMC-1

Abstract

We present a study of CH$_3$CH$_2$CCH, CH$_3$CH$_2$CN, CH$_2$CHCCH, and CH$_2$CHCN in TMC-1 using the QUIJOTE$^1$ line survey. We confirm the presence of CH$_3$CH$_2$CCH in TMC-1, which was previously reported as tentative by our group. From a detailed study of the ethynyl and cyanide derivatives of CH$_2$CH$_2$ and CH$_3$CH$_3$ in TMC-1, we found that the CH$_2$CHCCH/CH$_2$CHCN and CH$_3$CH$_2$CCH/CH$_3$CH$_2$CN abundance ratios are 1.5pm 0.1 and 4.8pm 0.5, respectively. The derived CH$_2$CHCCH/CH$_3$CH$_2$CCH abundance ratio is 15.3pm 0.8, and that of CH$_2$CHCN over CH$_3$CH$_2$CN is 48pm 5. All the single substituted isotopologs of vinyl cyanide have been detected, and we found that the first and second carbon substitutions in CH$_2$CHCN 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, CH$_2$CH13CN, presents an increase in its abundance similar to that found for HCCCN. Finally, we observed eight $b$-type transitions of CH$_2$CHCN, and we find that their intensity cannot be fitted adopting the dipole moment $ derived previously. These transitions involve the same rotational levels as those of the $a$-type transitions. From their intensity, we obtain $ 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 CH$_3$CH$_2$CCH, CH$_3$CH$_2$CN, CH$_2$CHCCH, and CH$_2$CHCN observed in TMC-1 can be explained in terms of gas-phase reactions.