DETECTION OF A NEW MOLECULAR ION HC3NH-1( IN TMC)

Protonated cyanoacetylene HC3NH+ has been detected toward TMC-1 for th e first time with the Nobeyama 45 m radiotelescope through the J = 5-4 and J = 4-3 transitions. This observation provides the first measurem ent of the pure rotational transitions for this ion either in the labo ratory or in space. From a simultaneous analysis of rotational transit ion frequencies obtained in this work and the previous infrared data, the most accurate rotational and the centrifugal distortion constants were determined to be B = 4329.0047(10) and D = 0.000478(11) MHz, resp ectively, with one standard deviation in parentheses. The column densi ty of HC3NH+ in TMC-1 (cyanopolyyne peak) was determined to be 1.0 (0. 2) x 10(12) cm-2 which is 160 times smaller than that of HC3N and 2.6 times more abundant than HNCCC. The abundances of HC3NH+, HNCCC, and H CCCN are discussed on the basis of a reaction mechanism recently propo sed for the production of HCCCN through neutral-neutral reactions.