Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The Aliphatic C--D Band Strengths

Deuterium (D) was exclusively generated in the Big Bang and the standard Big Bang Nucleosynthesis (BBN) model predicts a primordial abundance of D/H~26ppm. As the Galaxy evolves, D/H gradually decreases because of astration. The Galactic chemical evolution (GCE) model predicts a present-day abundance of D/H~20ppm. However, observations of the local interstellar medium (ISM) have revealed that the gas-phase interstellar D/H varies considerably from one region to another and has a median abundance of D/H~13ppm, substantially lower than predicted from the BBN and GCE models. It has been suggested that the missing D atoms of D/H~7ppm could have been locked up in deuterated polycyclic aromatic hydrocarbon (PAH) molecules. However, we have previously demonstrated that PAHs with aromatic C–D units are insufficient to account for the missing D. Here we explore if PAHs with aliphatic C–D units could be a reservoir of D. We perform quantum chemical computations of the vibrational spectra of “superdeuterated” PAHs (in which one D and one H share an C atom) and PAHs attached with D-substituted methyl group, and derive the band strengths of the aliphatic C–D stretch (A_4.65). By applying the computationally derived A_4.65 to the observed aliphatic C–D emission at ~4.6–4.8 micron, we find that PAHs with aliphatic C–D units could have tied up a substantial amount of D/H and marginally account for the missing D. The possible routes to generate PAHs with aliphatic C–D units are also discussed.