Time-Dependent Density Functional Study of Nitrogen-Substituted Polycyclic Aromatic Hydrocarbons and Diffuse Interstellar Bands

Abstract

This work reports theoretical calculations of electronic transitions in nitrogen-substituted polycyclic aromatic hydrocarbon neutrals and cations, using time-dependent density functional theory. The results obtained are compared with the diffuse interstellar bands, a broad group of absorption bands that can be seen mostly in near-ultraviolet and near-infrared wavelengths of the spectrum. It is observed that with nitrogen substituted at the periphery (exoskeletal), these nitrogen-substituted polycyclic aromatic hydrocarbon neutrals and their cation counterparts, similar to their corresponding parent polycyclic aromatic hydrocarbons, absorb in the near-ultraviolet and near-infrared wavelengths, respectively. The analogy then follows a change with nitrogen entering into the structure (endoskeletal) and the nitrogen-substituted polycyclic aromatic hydrocarbon neutrals and cations, unlike their corresponding pure polycyclic aromatic hydrocarbon family, fall in the near-infrared and visible spectral regions, respectively. Based on these and other astrophysical implications, it is concluded that nitrogen-substituted polycyclic aromatic hydrocarbons represent a powerful class of prospective carriers of diffuse interstellar bands. © 2022 American Chemical Society.