Kekulene: Investigating its Vibrational Modes in the Circumstellar Medium

Abstract

Aims/Objectives: To investigate the vibrational properties of Kekulene (C48H24), a large, highly symmetric polycyclic aromatic hydrocarbon (PAH), and evaluate its potential as a carrier of the Aromatic Infrared Bands (AIBs) observed in circumstellar and interstellar environments. The work aims to assess Kekulene’s astrochemical relevance and diagnostic utility in modeling PAH populations in space. Methodology: Density Functional Theory (DFT) calculations were performed using the GAMESS quantum chemistry package. The B3LYP/6-31G(d) level of theory is employed to optimize geometries and compute infrared (IR) frequencies. For the neutral molecule, restricted Hartree–Fock (RHF) was used, while the unrestricted Hartree–Fock (UHF) method was applied to the cation. Mulliken population analysis is used to assess the electronic charge and its redistribution upon ionization. Simulated IR spectra were constructed using Lorentzian profiles with 5 cm−1 full width at half maximum (FWHM). Results: Kekulene exhibits distinct vibrational features attributed to its unique ring topology and superaromatic character. The neutral form shows strong CH out-of-plane bending at 860.9 and 884.4 cm−1 and prominent CH stretching near 3096.7 cm−1, comparing well with the observed 11.2 µm and 3.3 µm AIBs, respectively. Upon ionization, charge redistribution enhances the intensity of CC stretching and CH in-plane bending modes, from 1600 to 1000 cm−1 region. The most intense peak in the cation is at 1221.1 cm−1, close to the 8.2 µm AIB. Comparison with compact Circumcoronene points to unique spectral signatures of Kekulene’s inner hydrogen modes. Conclusion: The results suggest that Kekulene, especially in its cationic form, could significantly contribute to the composite PAH emission observed in UV-rich astrophysical environments.