Vibrational and Electronic Spectroscopy of 2-Cyanoindene Cations

Authors

Thomas E. Douglas-Walker, University of Edinburgh
Eleanor K. Ashworth, University of East Anglia
Mark H. Stockett, Stockholm University
Francis C. Daly, University of Edinburgh
Isabelle Chambrier, University of East Anglia
Vincent J. Esposito, Chapman UniversityFollow
Marius Gerlach, Radboud University
Angel Zheng, University of Edinburgh
Julianna Palotás, University of Edinburgh
Andrew N. Cammidge, University of East Anglia
Ewen K. Campbell, University of Edinburgh
Sandra Brünken, Radboud University
James N. Bull, University of East Anglia

Document Type

Article

Publication Date

12-16-2024

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 D2 ← D0 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 D₂ ← D₀ 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.

Comments

This article was originally published in ACS Earth and Space Chemistry, volume 9, issue 1, in 2025. https://doi.org/10.1021/acsearthspacechem.4c00270

Recommended Citation

Douglas-Walker T., Ashworth E. K., Stockett M. H., Daly F., Chambrier I., Esposito V. J., Gerlach M., Cammidge A. N., Campbell E. K., Brünken S., Bull J. N. “Vibrational and Electronic Spectroscopy of 2-Cyanoindene Cations.” ACS Earth Space Chem. 9, 134 (2025). https://doi.org/10.1021/acsearthspacechem.4c00270

Copyright

The authors

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This work is licensed under a Creative Commons Attribution 4.0 License.