Asymmetric polycyclic aromatic hydrocarbon as a capable source of astronomically observed interstellar infrared spectrum

TL;DR

This study uses density functional theory to analyze asymmetric polycyclic aromatic hydrocarbons (PAHs) and finds certain asymmetric cationic PAHs match well with observed interstellar infrared spectra, suggesting they are likely sources of interstellar dust.

Contribution

It demonstrates that asymmetric and cationic PAHs can better explain interstellar IR spectra than symmetrical molecules, offering new insights into interstellar dust composition.

Findings

Asymmetric cationic PAHs show good IR spectral matches with astronomical observations.

Symmetrical PAHs have little resemblance to observed IR spectra.

C12H83+ exhibits an excellent match with observed IR wavelengths.

Abstract

In order to find out capable molecular source of astronomically well observed infrared (IR) spectrum, asymmetric molecular configuration polycyclic aromatic hydrocarbon (PAH) was analyzed by the density functional theory (DFT) analysis. Starting molecules were benzene C6H6, naphthalene C10H8 and 1H-phenalene C13H9. In interstellar space, those molecules will be attacked by high energy photon and proton, which may bring cationic molecules as like C6H6n+ (n=0~3 in calculation), C10H8n+, and C13H9n+, also CH lacked molecules C5H5n+, C9H7n+, and C12H8n+. IR spectra of those molecules were analyzed based on DFT based Gaussian program. Results suggested that symmetrical configuration molecules as like benzene, naphthalene , 1H-phenalene and those cation ( +, 2+, and 3+) show little resemblance with observed IR. Contrast to such symmetrical molecules, several cases among cationic and asymmetric configuration molecules show fairly good IR tendency. One typical example was C12H83+, of which calculated harmonic IR wavelength were 3.2, 6.3, 7.5, 7.8, 8.7, 11.3, and 12.8 micro meter, which correspond well to astronomically observed wavelength of 3.3, 6.2, 7.6, 7.8, 8.6, 11.2, and 12.7 micro meter. It was amazing agreement. Also, some cases like C5H5+, C9H7+, C9H72+, C9H73+ and C12H82+ show fairly good coincidence. Such results suggest that asymmetric and cationic PAH may be capable source of interstellar dust.