Anharmonic Infrared Signatures of C ₉ H ₇ CN Isomers: CN Stretching as a Probe of Cyano-PAHs in the 4.3–4.5 μm Region

Recent radio detections of cyano-substituted aromatic molecules have established nitrile functionalization as a powerful observational auxiliary for identifying polycyclic aromatic hydrocarbons (PAHs) in space. A comprehensive theoretical study of 189 possible nitrile isomers of the molecular formula C 9 H 7 7 CN has been performed to explore the rotational and vibrational spectroscopic signatures using dispersion-corrected density functional theory (DFT). The cyanoindene derivatives dominate the low-energy region of the potential energy surface, with 2-cyanoindene identified as the global minimum. The anharmonic vibrational spectra have been computed using the second-order vibrational perturbation theory with explicit treatment of resonance polyads. Computed composite anharmonic spectra of the seven low-lying isomers show a cluster of strong and confined absorption bands at 4.367 μm with a narrow width of 0.067 μm originating from the −CN stretching. The intense anharmonic coupling of the −CN stretch with proximal combination vibrations manifests as a distinct, structured spectral band due to Fermi resonances. This spectroscopic signature closely mirrors the narrow, high-contrast feature predicted for larger cyano-PAHs and provides a natural explanation for the structured baseline and broad emission features observed around ∼4.3–4.4 μm in recent near-infrared spectra (NIRSpec) on the James Webb Space Telescope (JWST).