A new protonated molecule discovered in TMC-1: HCCNCH+

TL;DR

This paper reports the first detection of the protonated metastable isomer HCCNCH+ in TMC-1, using radio telescopes, supported by high-level ab initio calculations, and discusses its abundance and chemical implications in cold dense clouds.

Contribution

The discovery of HCCNCH+ as the first protonated metastable isomer in a cold dense cloud, supported by observational data and theoretical modeling.

Findings

HCCNCH+ detected with a column density of (3.0 +/- 0.5)e10 cm-2

HCCNCH+/HCCNC abundance ratio of 0.010 +/- 0.002

The observed ratio aligns with chemical model predictions

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 40m and IRAM 30m telescopes, which revealed four harmonically related lines. We derive 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 assign the series of lines to the ion HCCNCH+. We derive a column density of (3.0 +/- 0.5)e10 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 restricts to species M with high proton affinities (>700 kJ/mol), MH+/M ratios fall in the range 0.001-0.1, with no apparent correlation with proton affinity. We suggest various protonated molecules that are good candidates for detection in cold dense clouds in the near future.