Investigating C−D out-of-plane vibrational modes in PAHs as a tool to study interstellar deuterium-containing PAHs

ABSTRACT Previous as well as recent observations by ISO, Spitzer, AKARI, SOFIA, JWST etc. have revealed various characteristics of mid-infrared emission bands between 3 and 20 $\mu$m. Subsequently, several forms of organics including polycylic aromatic hydrocarbons (PAHs)/PAH-like molecules are proposed as carriers for these bands. Deuterated PAH (PAD) is one such substituted PAH, which is proposed as a potential candidate carrier for weak emission bands at 4.4 and 4.65 $\mu \rm m$, detected towards few astronomical targets and are characteristics of aromatic and aliphatic C–D stretching modes in a PAD molecule, respectively. However, the 4.4 $\mu$m band is not widely detected. In order to validate PADs as carriers for mid-infrared emission bands, an additional alternative tool is crucial. If PAHs are deuterated, they should also possess an inherent signature from the C–D out-of-plane (C–D$\rm _{oop}$) vibrations, which are at the longer wavelength side. In this report, features due to C–D$\rm _{oop}$ modes in PAHs bearing a single to multiple deuterium atoms are reported by performing quantum-chemical calculations. This paper reports that some of the C–D$\rm _{oop}$ vibrations appear at the 14–19 $\mu$m range. Also, the strength of C–D$\rm _{oop}$ modes is not proportional to the D/H ratio in PAHs. In addition, a moderate change in the spectra of deuterated PAHs is observed from that of the undeuterated counterparts, as deuteration would alternate the adjacency class of the C-H bonds and the mass distribution of the molecule. We discuss the efficiency and usefulness of these bands to constrain the form of PAHs emitting mid-infrared emission bands.