2023/08/08 by Xuejuan Yang, Aigen Li, Yang, X. J. +1 · 1 citation
Chemistry · Pharmacology, Toxicology and Pharmaceutics · Physics and Astronomy · #Advanced Chemical Physics Studies #Astrophysics of Galaxies (astro-ph.GA) #Chemical Reactions and Isotopes #FOS: Physical sciences #Molecular Spectroscopy and Structure #Solar and Stellar Astrophysics (astro-ph.SR)
paper · pdf · doi:10.48550/arxiv.2308.04085
openalex publication_date 2023/08/08 · openalex created_date 2023/08/10 · openalex updated_date 2026/07/28
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 (A4.65). By applying the computationally derived A4.65 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.