Hyperfine Structure Identification of Interstellar Cyanoallene toward TMC-1

Interstellar cyanoallene (CH₂CCHCN) has been detected with the 100 m Green Bank Telescope (GBT) toward the dark Taurus molecular cloud (TMC-1) by means of the 4₁₄-3₁₃, 4₀₄-3₀₃, 5₁₅-4₁₄, and 5₀₅-4₀₄ rotational transitions at 20.2, 20.6, 25.2, and 25.7 GHz, respectively. The hyperfine structure (hfs) of the 4₁₄-3₁₃ transition is resolved; the remaining transitions have hfs components that are blended but are inferred owing to line width excesses atypical of TMC-1. An analysis of these four rotational transitions yields an average column density N_T = 2.0(5) × 10¹² cm^-2 at an assumed rotational temperature of 4 K. The cyanoallene isomer methylcyanoacetylene (CH₃CCCN) has previously been observed in TMC-1 and found to have a rotational temperature of ~4 K by other investigators. Using the GBT, the K = 0 and K = 1 structure of the J_K = 5_K-4_K transition of methylcyanoacetylene was resolved toward TMC-1 and a total column density of N_T = 4.5(4) × 10¹¹ cm^-2 was obtained for the 4 K rotational temperature. Thus, cyanoallene appears to be more abundant than methylcyanoacetylene toward TMC-1 by a factor of ~4.5. The implications of this result are discussed in terms of relative bonding energy differences and possible differences in formation chemistry.