Rotational Spectra of Small PAHs: Acenaphthene, Acenaphthylene, Azulene, and Fluorene

Pure rotational spectra of four small polycyclic aromatic hydrocarbons have been observed by Fourier transform microwave spectroscopy of a molecular beam in the frequency range from 7 to 37 GHz. Initial searches for acenaphthene (C12H10), acenaphthylene (C12H8), and fluorene (C13H10) were guided by quantum chemical calculations performed at the B3LYP/cc-pVTZ level of theory. All three molecules exhibit b-type rotational spectra and are calculated to be moderately polar, with dipole moments of 0.3Y0.9 D. Close agreement (to better than 1%) between the calculated equilibrium and experimentally derived ground-state rotational constants is achieved. Selected transitions of acenaphthene and fluorene have also been measured in the 3 mm region by conventional free-space absorption spectroscopy, as have transitions of the previously studied azulene (C10H8). The data presented here facilitate deep radio astronomical searches with large radio telescopes. Subject headinggs: astrochemistry — ISM: molecules — molecular data Online material: machine-readable tables