The radio spectra of planar aromatic heterocycles: how to quantify and predict the negative inertial defects

The simplest tricyclic aromatic nitrogen heterocyclic molecules 5,6-benzoquinoline and 7,8-benzoquinoline are possible candidates for detection of aromatic systems in the interstellar medium. Therefore the pure rotational spectra have been recorded using frequency-scanned Stark modulated, jet-cooled millimetre wave absorption spectroscopy (48–87 GHz) and Fourier Transform Microwave (FT-MW) spectroscopy (2–26 GHz) of a supersonic rotationally cold molecular jet. Guided by theoretical molecular orbital predictions, spectral analysis of mm-wave spectra, and higher resolution FT-MW spectroscopy provided accurate rotational and centrifugal distortion constants together with 14N nuclear quadrupole coupling constants for both species. The tricyclic frames of these species undergo low energy out-of-plane zero-point vibrations resulting in deviations from the moments of inertia that the rigid structure would exhibit. The determined inertial defects, along with those of similar species are used to develop an empirical formula for calculation of inertial defects of aromatic ring systems. The predictive ability of the formula is shown to be excellent in general for planar species with a number of pronounced out-of-plane vibrations. The resultant constants for the benzoquinolines are of sufficient accuracy to be used in astrophysical searches for planar aromatic heterocycles.