Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications

Abstract

The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 $\mu$m ubiquitously seen in a wide variety of astrophysical regions are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. Astronomical PAHs may have an aliphatic component as revealed by the detection in many UIE sources of the aliphatic C-H stretching feature at 3.4 $\mu$m. The ratio of the observed intensity of the 3.4 $\mu$m feature to that of the 3.3 $\mu$m aromatic C-H feature allows one to estimate the aliphatic fraction of the UIE carriers. This requires the knowledge of the intrinsic oscillator strengths of the 3.3 $\mu$m aromatic C-H stretch ($A_{3.3}$) and the 3.4 $\mu$m aliphatic C-H stretch ($A_{3.4}$). Lacking experimental data on $A_{3.3}$ and $A_{3.4}$ for the UIE candidate materials, one often has to rely on quantum-chemical computations. Although the second-order Moller-Plesset (MP2) perturbation theory with a large basis set is more accurate than the B3LYP density functional theory, MP2 is computationally very demanding and impractical for large molecules. Based on methylated PAHs, we show here that, by scaling the band strengths computed at an inexpensive level (e.g., B3LYP/6-31G*) we are able to obtain band strengths as accurate as that computed at far more expensive levels (e.g., MP2/6-311+G(3df,3pd)).

25 pages, 9 figures; accepted for publication in The Astrophysical Journal

References in corpus (13)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
• Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Constraints from the Interstellar C-H Stretching Features at 3.2-3.5 Micrometers
• The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons
• Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
• AKARI near-infrared spectroscopy of the aromatic and aliphatic hydrocarbon emission features in the galactic superwind of M 82
• Search for the Infrared Emission Features from Deuterated Interstellar Polycyclic Aromatic Hydrocarbons
• The C--H Stretching Features at 3.2--3.5 Micrometer of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• Observational studies on the Near-Infrared Unidentified Emission Bands in Galactic HII regions
• On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
• On the relevance of polyynyl-substituted PAHs to astrophysics

Cited by in corpus (12)

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• PHANGS-JWST First Results: Mapping the 3.3 micron Polycyclic Aromatic Hydrocarbon Vibrational Band in Nearby Galaxies with NIRCam Medium Bands
• The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared
• DFT study on interstellar PAH molecules with aliphatic side groups
• A New Technique for Measuring Polycyclic Aromatic Hydrocarbon Emission in Different Environments
• Probing computational methodologies in predicting mid-infrared spectra for large polycyclic aromatic hydrocarbons
• Interstellar dehydrogenated PAH anions: vibrational spectra
• Orion Bar as a window to the evolution of PAHs
• Machine-learning identified molecular fragments responsible for infrared emission features of polycyclic aromatic hydrocarbons
• Detection of Deuterated Hydrocarbon Nanoparticles in the Whirlpool Galaxy, M51
• In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C${}_2$H${}_2$ ice