Locally Aromatic Polycyclic Hydrocarbons as Potential Carriers of Infrared Emission Features

Date

2003

Authors

Petrie, Simon
Stranger, Robert
Duley, Walter

Journal Title

Journal ISSN

Volume Title

Publisher

IOP Publishing

Abstract

We report B-LYP/6-31G* and B3-LYP/6-31G* density functional theory calculations on a set of polycyclic hydrocarbons, ranging in size from C19H22 to C36H32, combining aromatic (unsaturated) and aliphatic (saturated, sp3-hybridized carbon) ring systems. These locally aromatic polycyclic hydrocarbons (LAPHs), generally exhibiting large deviations from planarity, may be considered as intermediate structures between polycyclic aromatic hydrocarbons (PAHs) and nanodiamonds. Calculated infrared vibrational frequencies are found to be similar to those observed experimentally in spectra of hydrogenated amorphous carbon (HAC) and other carbonaceous solids. In the C-H stretching region (∼3.1-3.6 μm) these species are characterized by strong absorption/emission within both the aliphatic and aromatic C-H bands. They also show spectral features associated with tertiary C-H. Similar features are evident in calculated spectra of the corresponding ions, which we have characterized in some cases. Ionization results in the particular enhancement of a spectral feature typically seen at ∼6.4 μm, in the aromatic C-C stretching region. In keeping with previous experimental and theoretical studies on the spectra of neutral and cationic PAHs, we find that the influence of ionization on the relative intensities of C-C and C-H stretching features is much greater than the influence of molecular structure. We suggest that LAPHs may be significant contributors to emission in Type B unidentified infrared emission sources.

Description

Keywords

Keywords: Astrochemistry; Circumstellar matter; Dust, extinction; Infrared: ISM; ISM: molecules; Molecular data

Citation

Source

Astrophysical Journal, The

Type

Journal article

Book Title

Entity type

Access Statement

License Rights

DOI

10.1086/377119

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