Rotational spectra of small PAHs

Initial searches for acenaphthene (C_12_H_10_), acenaphthylene (C_12_H_8_), and fluorene (C_13_H_10_) 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 0.3-0.9D.

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 3mm region by conventional free-space absorption spectroscopy, as have transitions of the previously studied azulene (C_10_H_8_). The data presented here facilitate deep radio astronomical searches with large radio telescopes.