FT‐MW and Millimeter Wave Spectroscopy of PANHs: Phenanthridine, Acridine, and 1,10‐Phenanthroline

Abstract

The pure rotational spectra of phenanthridine, acridine, and 1,10-phenanthroline, small polycyclic aromatic nitrogen heterocycle molecules (PANHs), have been measured and assigned from 2 to 85 GHz. An initial spectral assignment, guided by ab initio molecular orbital predictions, employed broadband Stark modulated millimeter wave absorption spectroscopy of a supersonic rotationally cold molecular beam, yielding a preliminary set of rotational and centrifugal distortion constants. Subsequent spectral analysis employed Fourier transform microwave (FT-MW) spectroscopy of a supersonic rotationally cold molecular beam. The extremely high spectral resolution of the FT-MW instrument yielded improved rotational constants and centrifugal distortion constants, together with nitrogen quadrupole coupling constants, for all three species. Density functional theory (DFT) calculations at the B3LYP level of theory employing the cc-pVTZ and 6-311+G** basis sets are shown to closely predict rotational constants and to be useful in predicting quadrupole coupling constants and dipole moments for such PANH species. The data presented here will be useful for deep radio astronomical searches for PANHs employing large radio telescopes.