Heteroatom effects on tricyclic fused aromatic molecules revealed by rotational spectroscopy.

Abstract

The gas-phase structures of dibenzothiophene, dibenzofuran, and carbazole provide detailed insights into how heteroatomic substitution changes the structural and electronic characteristics of tricyclic aromatic systems. Here we investigated the molecular properties such as rotational constants and the geometry of the three compounds in a supersonic expansion using high-resolution chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in a recently built spectrometer operating in the 2-18 GHz frequency range. Assisted by ab initio (MP2) and density functional theory (B3LYP-D3(BJ)) quantum-chemical calculations, the spectral analysis allowed us to assign the parent species and the heavy-atom isotopologues in natural abundance (13C and either 15N,34S, or 18O) for all three molecules. The isotopic data enabled the determination of the experimental structure through several methods, namely the substitution structure (rs), the vibrational ground-state geometry (r0), the semi-experimental equilibrium structure (rSEe), and the first-order mass-dependent r(1)m structure, through analysis of the changes in the principal moments of inertia induced by single-atom isotopic substitution. The experimental structures of these three heteroatom-containing PAHs are compared with the previously reported fluorene molecule. Finally, we discuss the underlying structural and electronic changes caused by the heteroatoms in the fluorene scaffold.

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