Theoretical study of infrared and ultraviolet spectra of 14 isomers of C24 and comparison with astronomical observations

Abstract

The present paper discusses the infrared features of C24 based on the Density Functional Theory (DFT) calculation and suggests some of the features observed in celestial objects may be attributed to C24. We also calculate the electronic absorption spectra of the C24 isomers to compare with the bump feature at 217 nm in the interstellar extinction curve. The C24 isomers are of four groups viz. cage, planar, bowl, and ring forms, and the present study considers their neutral and charged states. The structural parameters are reported for the first time. The planar structure is the most stable and the ring structure has a significant dipole moment observed. We extract theoretical infrared spectra of fourteen isomers in their neutral and charged states at the B3LYP /6-311++G** level of theory. The time-dependent density functional theory (TDDFT) approach is used to calculate the electronic transitions, the absorbance, and the HOMO to LUMO gaps of the 14 C24 isomers in their neutral and charged states. Upon ionization, significant changes are observed in the infrared and electronic absorption spectra, and the structural parameters. Average theoretical spectra of the cage, planar, bowl, and ring of the C24 isomer show the features at 6.2 μm, 7.65 μm, 8.65 μm, 11.3 μm, 12.8 μm, and 35.6 μm, which match with the features in the observed spectra of the reflection nebulae, NGC 2023 and NGC 7023. A sign of a bump in the ultraviolet at around 218 nm is observed in the electronic absorption spectra.