Authors: Agundez, M.; Cabezas, C.; Marcelino, N.; Fuentetaja, R.; Tercero, B.; de Vicente, P.; Cernicharo, J.


Publication date: 2022/03/24

DOI: 10.1051/0004-6361/202243396

Abstract: In recent years we have seen an important increase in the number of protonated molecules detected in cold dense clouds. Here we report the detection in TMC-1 of HCCNCH+, the protonated form of HCCNC, which is a metastable isomer of HC3N. This is the first protonated form of a metastable isomer detected in a cold dense cloud. The detection was based on observations carried out with the Yebes 40 m telescope and the 30 m telescope of the Institut de Radioastronomie Millimetrique (IRAM), which revealed four harmonically related lines. We derived a rotational constant B = 4664.431891 +/- 0.000692 MHz and a centrifugal distortion constant D = 519.14 +/- 4.14 Hz. From a high level ab initio screening of potential carriers, we confidently assigned the series of lines to the ion HCCNCH+. We derived a column density of (3.0 +/- 0.5) x 10(10) cm(-2) for HCCNCH+, which results in a HCCNCH+/HCCNC abundance ratio of 0.010 +/- 0.002. This value is well reproduced by a state-of-the-art chemical model which, however, is subject to important uncertainties regarding the chemistry of HCCNCH+. The observational and theoretical status of protonated molecules in cold dense clouds indicate that there exists a global trend in which protonated-to-neutral abundance ratios MH+/M increase with increasing proton affinity of the neutral M, although if one is restricted to species M with high proton affinities (> 700 kJ mol(-1)), MH+/M ratios fall in the range 10(-3)-10(-1), with no apparent correlation with the proton affinity. We suggest various protonated molecules that are good candidates for detection in cold dense clouds in the near future.