Theoretical study of aromatic and N-heterocycles: Potential carriers of interstellar aromatic infrared bands

Heterocycle compounds that contain nitrogen play important roles in astrobiology and terrestrial biology as a building block of nucleic acid. Nitrogen inclusion in polycyclic aromatic hydrocarbons (PAHs) is also proposed to account for the emission band at 6.2 µm observed in the interstellar medium (ISM). This work comprehensively studies the theoretical microwave, infrared (IR), and ultraviolet (UV)/visible spectra of aromatic and N-heterocycles in the gas phase and water solvent. In this study, one tricyclic tetrahydroquinoline (THQ) and two fused polycyclic quinolines, and their carboxylic acid substitutions in their neutral and ionic states are reported for the first time. All the calculations are performed at the B3LYP/aug-cc-pVTZ level of theory using the Gaussian 16. The rotational constants are obtained at this level of theory, which are further used to calculate the rotational spectra and their transitions. The IR spectra of these molecules are also reported to discuss possible carriers of the aromatic infrared bands (AIBs) observed in the ISM. The electronic absorption spectra of the molecule are also obtained using the time-dependent density functional theory (TD-DFT) at the same level of theory, allowing us to determine the energy gap between the Highest Occupied Molecular Orbitals (HOMO) and Lowest Unoccupied Molecular Orbitals (LUMO) and discuss their stability. We further estimate the transition wavelength, oscillator strength, and symmetry using the AOMix. This study contributes to understanding the physicochemical properties of the building blocks of genetic molecules.