The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR

Vincent J Esposito; Piero Ferrari; Wybren Jan Buma; Ryan C Fortenberry; Christiaan Boersma; Alessandra Candian; Alexander G G M Tielens

doi:10.1063/5.0191404

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR

J Chem Phys. 2024 Mar 21;160(11):114312. doi: 10.1063/5.0191404.

Authors

Vincent J Esposito¹, Piero Ferrari², Wybren Jan Buma^{2

3}, Ryan C Fortenberry⁴, Christiaan Boersma¹, Alessandra Candian⁵, Alexander G G M Tielens^{6

7}

Affiliations

¹ NASA Ames Research Center, MS 245-6, Moffett Field, California 94035, USA.
² Radboud University, Institute for Molecules and Materials, HFML-FELIX, 6525 ED Nijmegen, The Netherlands.
³ Van't Hoff Institute for Molecular Sciences, University of Amsterdam, 1098 XH Amsterdam, The Netherlands.
⁴ Department of Chemistry and Biochemistry, University of Mississippi, University, Mississippi 38677-1848, USA.
⁵ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam, The Netherlands.
⁶ Leiden Observatory, Leiden University, 2333 CA Leiden, The Netherlands.
⁷ Astronomy Department, University of Maryland, College Park, Maryland 20742-2421, USA.

PMID: 38501470
DOI: 10.1063/5.0191404

Abstract

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm-1 for d1-phenylacetylene, a one-to-one match is seen when comparing computations and experiments with all features assigned to combination bands and overtones. The C≡C stretch observed in phenylacetylene is not observed in d1-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.