Vetting molecular candidates posited for the first diffuse interstellar bands (5780 and 5797 Å): a quantum chemical study

Abstract

Diffuse interstellar bands (DIBs) comprise over 550 celestial absorption features whose molecular carriers remain largely unidentified or contested. In this study, we present a statistical analysis that identifies two previously overlooked families of strongly correlated lines associated with the original Heger features at 5780 and 5797 Å. Comprehensive UV-vis spectra were computed at several levels of theory (mainly TD-PBE0 and EOM-CCSD with an aug-cc-pVTZ basis set) for the following candidates posited as diffuse interstellar band carriers (in both their neutral and cationic forms): 2-cyclopenten-1-one, 3(2H)-thiophenone, 2(5H)-furanone, 3(2H)-selenophenone, 3-hydroxypropanamide, oxamic acid, lactamide, and glycolamide. Glycolamide is of particular interest since it has recently been detected in microwave (rotational) spectra of the comparatively dense molecular cloud G+0.693–0027. Importantly, the computations reveal that the anions exhibit marginal electron affinities despite producing improbable lines (i.e., with excitations to levels above the ionization threshold) overlapping DIBs, whereas the neutral molecules yield lines shortward of DIBs and possibly linked to the broad 220 nm interstellar feature, and their cations produced too few lines in the DIB domain inspected. Further vetting of candidates awaits the construction of an expansive optical-infrared molecular ion database, which will facilitate concurrent matching to DIBs in the optical (electronic) and their energy differences in the mid-infrared (vibrational), thereby narrowing the parameter space.