Hyperfine Structure Identification of Interstellar Cyanoallene toward TMC-1

Interstellar cyanoallene (CH sub(2)CCHCN) has been detected with the 100m Green Bank Telescope (GBT) toward the dark Taurus molecular cloud (TMC-1) by means of the 4 sub(14)-3 sub(13),4 sub(04)-3 sub(03), 5 sub(15)-4 sub(14), and 5 sub(05)-4 sub(04) rotational transitions at 20.2, 20.6, 25.2, and 25.7 GHz, respectively. The hyperfine structure (hfs) of the 4 sub(14)-3 sub(13) 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 sub(T) = 2.0(5) x 10 super(12) cm super(-2) at an assumed rotational temperature of 4 K. The cyanoallene isomer methylcyanoacetylene (CH sub(3)CCCN) has previously been observed in TMC-1 and found to have a rotational temperature of 64 K by other investigators. Using the GBT, the K = 0 and K = 1 structure of the J sub(K) = 5 sub(K)-4 sub(K) transition of methylcyanoacetylene was resolved toward TMC-1 and a total column density of N sub(T) = 4.5(4) x 10 super(11) cm super(-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 64.5. The implications of this result are discussed in terms of relative bonding energy differences and possible differences in formation chemistry.