Toward the Detection of Cyanoketene in the Interstellar Medium: New Hints from Quantum Chemistry and Rotational Spectroscopy

A detailed quantum chemical investigation of a new reaction mechanism possibly leading to the formation of cyanoketene (NC-CH=C=O) in the interstellar medium (ISM) was carried out. Different reaction channels have been found by the AutoMeKin program, and the structures and harmonic force fields of the key stationary points have been characterized at the density functional theory level employing last-generation double-hybrid functionals. Finally, single-point compu-tations at those geometries by state-of-the-art composite wave function methods provided accurate energies for the evaluation of thermochemical and kinetic parameters in the framework of an Ab Initio Transition State Theory based Master Equation (AITSTME) strategy. Our results indicate that the barrier-less association reaction of the formyl radical (HCO center dot) to the cyanocarbene radical (HCCN) can lead to the formation of cyanoketene under the harsh conditions of the ISM. Canonical rate constants computed for temperatures up to 600 K show that the most abundant product is indeed cyanoketene. The formation of other, even more stable, species involves higher activation energies and/or less favorable multi-step processes. Furthermore, to aid the search of cyanoketene, still undetected in the ISM, its rotational spectrum was recorded up to 530 GHz. The refined set of spectroscopic constants obtained in this way allows for spectral predictions from the microwave to the terahertz region, particularly for the bright b-type transitions, which can be targeted for the identification of cyanoketene in spectral line surveys. Despite cyanoketene was already sought without success in a variety of astronomical sources, we suggest to look for it in those sources where HCO or HCCN have already been detected, namely, W3, NGC2024, W51, K3-50, IRC + 2016, and TMC-1.

Automated summary

In this study, a detailed quantum chemical investigation of a new reaction mechanism leading to the formation of cyanoketene in the interstellar medium (ISM) was conducted. Different reaction channels were discovered and characterized at the density functional theory level. The results indicate that the barrier-less association reaction of the formyl radical to the cyanocarbene radical can lead to the formation of cyanoketene under the harsh conditions of the ISM. The study also suggests potential astronomical sources where cyanoketene can be detected based on the presence of HCO and HCCN.