Detection of a New Molecular Ion HC 3NH + in TMC-1

Protonated cyanoacetylene HC3NH(+) has been detected toward TMC-1 for the first time with the Nobeyama 45 m radiotelescope through the J = 5-4 and J = 4-3 transitions. This observation provides the first measurement of the pure rotational transitions for this ion either in the laboratory or in space. From a simultaneous analysis of rotational transition 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, respectively, with one standard deviation in parentheses. The column density of HC3NH(+) in TMC-1 (cyanopolyyne peak) was determined to be 1.0 (0.2) x 10¹²/sq cm which is 160 times smaller than that of HC3N and 2.6 times more abundant than HNCCC. The abundances of HC3NH(+), HNCCC, and HCCCN are discussed on the basis of a reaction mechanism recently proposed for the production of HCCCN through neutral-neutral reactions.