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

Abstract

Pure rotational spectra of four small polycyclic aromatic hydrocarbons have been observed by Fourier transform microwave spectroscopy of amolecular beamin the frequency range from7 to 37GHz. 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 equilibriumand 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