| Abstract : | Annulenes are interesting derivatives of polycyclic aromatic hydrocarbon (PAH) molecules that can have aromatic as well as non-aromatic character. The ubiquitous presence of infrared emission features at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 micron in various astrophysical environments and their profile variations indicate the presence of a variety of PAH molecules in space. Emission models are attempted to match all the emission bands simultaneously. These models require infrared properties of specific PAH molecules as input. To have a broad data base for this input, quantum chemical computations have been performed for n-Annulenes, which are non-benzenoid PAHs. DFT computations are performed using B3LYP functional in conjugation with 6-31Gd basis set through the GAMESS suit of programs. [14]-Annulene (Pyrene with no interior bonds), [18]-Annulene (Coronene with no interior bonds), and [22]-Annulene are studied. All the molecules are aromatic in nature.The computed IR spectra of each molecule in neutral and cation are reported.
In nutral molecules C-H streatch vibrations are dominant feature, just as in the corresponding plain PAHs like pyrene and coronene. There is very little difference due to the inner hydrogens in [14]-Annulene and [18]-Annulene. In the cations too the spectral variation is similar to plain PAH cations. Additional infrared features that appear in the spectra are identified. The possibility of Annulene formation in the ISM and their relation to different astrophysical conditions are discussed. |
