Visible absorption spectra of mass-selected cyanobenzene and iso-cyanobenzene cations in solid neon

We report the visible absorption spectra of mass-selected cyano-benzene (C₆H₅CN⁺) and iso-cyano benzene (C₆H₅NC⁺) cations isolated in solid neon at 3 K using the mass-selected matrix isolation spectroscopy technique. The strong absorptions observed between 500 and 580 nm are assigned to the overlapping B ²B₂ ← X ²B₁ and C ²B₁ ← X ²B₁ transitions of C₆H₅CN⁺, which are in good agreement with recent gas-phase He-tagging measurements and Franck-Condon simulations. The additional weak features are attributed to C₆H₅NC⁺, supported by the detection of a distinct NC stretching vibration at 2043.3 cm^-1 in the IR spectrum and corroborated by quantum chemical calculations. Analysis of the integrated IR intensities suggests that the iso-cyanobenzene cation comprises only ∼7-5% of the cyano-benzene cation population under our deposition conditions. The present work provides the first condensed-phase benchmark spectra of cyano-substituted benzene cations, expanding the spectroscopic database of aromatic cations under cryogenic conditions.