Benchmarking Astrochemistry Paradigms: Relative Absence of $\rm{C_6H_5CN^+}$ in the Diffuse ISM

TL;DR

This study reevaluates the presence of benzonitrile cation in the diffuse interstellar medium, suggesting its relative absence due to energetic and observational mismatches, and uses new spectra and computational methods to benchmark astrochemistry models.

Contribution

It provides a comprehensive framework combining experimental, observational, and computational data to assess the detectability of C6H5CN+ in the diffuse ISM, highlighting its likely absence.

Findings

C6H5CN+ is relatively absent in the diffuse ISM.

Recurrent fluorescence unlikely prevents dissociation.

Spectral and observational mismatches support absence.

Abstract

The detectability of $\rm{C_6H_5CN^+}$ (benzonitrile cation) in the diffuse ISM is re-evaluated. A holistic evidentiary framework suggests $\rm{C_6H_5CN^+}$ is relatively absent in the diffuse ISM owing to the following concurrently: a marginal intramolecular vibrational energy redistribution (IVR) favoring fragmentation, recurrent fluorescence being an improbable mechanism in this case to prevent dissociation, mismatches between observed DIBs and experimental results, and the hitherto absence of DIBs matching any similarly sized cations. The putative gap in bottom-up synthesis is reaffirmed (diffuse ISM), and although DIB sources are largely unknown, within a broader approach the lines can help benchmark astrochemistry paradigms. The results relied on new advantageous Daly et al. experimental spectra, an expanded observational DIB analysis (APO catalog), and complementary $\omega$B97X-D/cc-pVTZ computations.