Benchmarking Astrochemistry Paradigms: Relative Absence of C6H5CN+ in the Diffuse ISM

The detectability of C6H5CN+ (benzonitrile cation) in the diffuse ISM is re-evaluated. A holistic evidentiary framework suggests C6H5CN+ 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, unceasing photon strikes, 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.