Battle of the CH Motions: Aliphatic vs Aromatic Contributions to Astronomical PAH Emission and Exploration of the Aliphatic, Aromatic, and Ethynyl CH Stretches

Authors

Vincent J. Esposito, Chapman UniversityFollow
Salma Bejaoui, NASA Ames Research Center
Brant E. Billinghurst, Canadian Light Source Inc.
Christiaan Boersma, NASA Ames Research Center
Ryan C. Fortenberry, University of Mississippi
Farid Salama, NASA Ames Research Center

Document Type

Article

Publication Date

11-15-2024

Abstract

Strong anharmonic coupling between vibrational states in polycyclic aromatic hydrocarbons (PAH) produces highly mixed vibrational transitions that challenge the current understanding of the nature of the astronomical mid-infrared PAH emission bands. Traditionally, PAH emission bands have been characterized as either aromatic or aliphatic, and this assignment is used to determine the fraction of aliphatic carbon in astronomical sources. In reality, each of the transitions previously utilized for such an attribution is highly mixed with contributions from both aliphatic and aromatic CH motions as well as non-CH motions such as CC stretches. High-resolution gas-phase IR absorption measurements of the spectra of the aromatic molecules indene and 2-ethynyltoluene at the Canadian Light Source combined with high-level anharmonic quantum chemical computations reveal the complex nature of these transitions, implying that the use of these features as a marker for the aliphatic fraction in astronomical sources is not uniquely true or actually predictive. Further, the presence of aliphatic, aromatic, and ethynyl CH groups in 2-ethynyltoluene provides an internally consistent opportunity to simultaneously study the spectroscopy of all three astronomically important groups. Finally, this study makes an explicit connection between fundamental quantum mechanical principles and macroscopic astronomical chemical physics, an important link necessary to untangle the lifecycle of stellar and planetary systems.

Comments

This article was originally published in Monthly Notices of the Royal Astronomical Society, volume 535, in 2024. https://doi.org/10.1093/mnras/stae2588

Recommended Citation

Esposito V. J., Bejaoui S., Billinghurst B. E., Boersma C., Fortenberry R. C., Salama F. “Battle of the CH motions: aliphatic vs. aromatic contributions to astronomical PAH emission and exploration of the aliphatic, aromatic, and ethynyl CH stretches.” MNRAS 535, 3293 (2024). https://doi.org/10.1093/mnras/stae2588

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The authors

Creative Commons License

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