Discovery of Interstellar trans-cyanovinylacetylene (HC ≡ CCH = CHC ≡ N) and vinylcyanoacetylene (H₂C = CHC₃N) in GOTHAM Observations of TMC-1

We report the discovery of two unsaturated organic species, trans-(E)-cyanovinylacetylene and vinylcyanoacetylene, using the second data release of the GOTHAM deep survey toward TMC-1 with the 100 m Green Bank Telescope. For both detections, we performed velocity stacking and matched filter analyses using Markov Chain Monte Carlo simulations, and for trans-(E)-cyanovinylacetylene, three rotational lines were observed at low signal-to-noise (∼3σ). From this analysis, we derive column densities of 2 × 10¹¹ and 3 × 10¹¹ cm⁻² for vinylcyanoacetylene and trans-(E)-cyanovinylacetylene, respectively, and an upper limit of <2 × 10¹¹ cm⁻² for trans-(Z)-cyanovinylacetylene. Comparisons with G3//B3LYP semiempirical thermochemical calculations indicate abundances of the [H₃C₅N] isomers are not consistent with their thermodynamic stability, and instead their abundances are mainly driven by dynamics. We provide a discussion on how these species may be formed in TMC-1, with reference to related molecules like vinyl cyanide (CH₂ = CHC ≡ N). As part of this discussion, we performed the same analysis for ethyl cyanide (CH₃CH₂C ≡ N), the hydrogenation product of CH₂ = CHC ≡ N. This analysis provides evidence—at 4.2σ significance—of an upper limit to the column density of <4 × 10¹¹ cm⁻²; an order of magnitude lower than previous upper limits toward this source.