Astronomical Search and Astrochemical Study of Cyanoisoquinolines in TMC-1: Rotational Spectroscopy and Interstellar Investigation

Polycyclic aromatic nitrogen heterocycles (PANHs) represent an intriguing class of prebiotic molecules whose cosmic origins remain poorly understood. While there have been detections of several PANHs in carbonaceous meteorites, their apparent absence in the interstellar medium, where their pure carbon counterparts are widespread, raises intriguing questions regarding their formation and destruction pathways. Here, we present the laboratory rotational spectroscopy and astronomical search for all seven isomers of the cyanoisoquinoline (isoquinolinecarbonitrile) PANH family toward the dense core of the cold dark cloud TMC-1 using observations from the GOTHAM spectral line survey. We report nondetections for all seven isomers and upper limits on their column densities. We then discuss potential formation pathways, including the formation of isoquinoline through bottom-up, top-down, and grain-surface ice chemistry processes, followed by the addition of CN to isoquinoline. The nondetection of these species may reflect observational challenges such as the dilution of signal across isomers, or it may point to differences in the formation and destruction pathways of PANHs compared to PAHs. These findings illustrate the complexity of PANH formation and emphasize the need for continued investigation into the processes that link the organic material present in interstellar and meteoritic environments.