Vibrational and Electronic Spectroscopy of 2-Cyanoindene Cations.

Abstract

2-Cyanoindene is one of the few specific aromatic or polycyclic aromatic hydrocarbon (PAH) molecules positively identified in Taurus molecular cloud-1 (TMC-1), a cold, dense molecular cloud that is considered the nearest star-forming region to Earth. We report cryogenic mid-infrared (550-3200 cm^-1) and visible (16,500-20,000 cm^-1, over the D ₂ ← D ₀ electronic transition) spectra of 2-cyanoindene radical cations (2CNI⁺), measured using messenger tagging (He and Ne) photodissociation spectroscopy. The infrared spectra reveal the prominence of anharmonic couplings, particularly over the fingerprint region. There is a strong CN-stretching mode at 2177 ± 1 cm^-1 (4.593 μm), which may contribute to a broad plateau of CN-stretching modes across astronomical aromatic infrared band spectra. However, the activity of this mode is suppressed in the dehydrogenated (closed shell) cation, [2CNI-H]⁺. The IR spectral frequencies are modeled by anharmonic calculations at the B3LYP/N07D level of theory that include resonance polyad matrices, demonstrating that the CN-stretch mode remains challenging to describe with theory. The D ₂ ← D ₀ electronic transition of 2CNI⁺, which is origin dominated, occurs at 16,549 ± 5 cm^-1 in vacuum (6041.8 Å in air). There are no correspondences with reported diffuse interstellar bands.