Infrared Spectroscopy of Pentagon-Containing PAHs: Indenyl and Fluorenyl Anions and Indenyl Cation

Gabi Wenzel ^1,2, Miguel Jiménez-Redondo ³, Milan Ončák ⁴

⁰Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

The Journal of Physical Chemistry Letters +

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April 11, 2025

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Summary

Polycyclic aromatic hydrocarbon (PAH) anions, like indenyl and fluorenyl, show vibrational modes matching space

Area Of Science

Astrochemistry
Quantum Chemistry
Spectroscopy

Background

Polycyclic aromatic hydrocarbons (PAHs) are abundant molecules in space.
PAH ions are implicated in interstellar chemistry and infrared emission.
Understanding PAH ion spectra is key to interpreting astronomical observations.

Purpose Of The Study

To investigate the infrared spectra of indenyl and fluorenyl anions and indenyl cation.
To identify potential carriers of the 7.7 μm interstellar PAH emission band.
To explore the vibrational properties of PAH ions using advanced spectroscopic techniques.

Main Methods

Infrared predissociation (IRPD) spectroscopy was employed.
Experiments were conducted in a cryogenic 22-pole ion trap at the FELion beamline.
Density functional theory (DFT) computations and molecular dynamics simulations were used for spectral analysis.

Main Results

Key vibrational modes of indenyl and fluorenyl anions near 1300 cm⁻¹ were identified.
These anionic PAH modes align with the observed 7.7 μm interstellar emission band.
The spectrum of cationic indenyls presented complexities not fully resolved by standard DFT calculations.

Conclusions

Indenyl and fluorenyl anions are strong candidates for the carriers of the 7.7 μm PAH emission.
Further investigation, potentially including isomer identification, is needed for cationic indenyl spectra.
This study advances the understanding of PAH ion spectroscopy and its role in astrochemistry.