π–π Stacking and Structural Configurations in Aromatic Thiophene and Fluorobenzene Dimers Revealed by Rotational Spectroscopy

Understanding the structures and bonding interactions between aromatic systems is crucial for interpreting the properties of biological molecules and materials. In this study, we employed chirp-polarization Fourier-transform microwave spectroscopy to investigate the structures and noncovalent interactions in both homogeneous and heterogeneous dimers formed by thiophene and fluorobenzene molecules. Our findings reveal that the thiophene dimer adopts a T-shaped structure, while the fluorobenzene dimer predominantly exhibits a parallel-displaced arrangement. For the heterogeneous dimer, both T-shaped and parallel-displaced structures are observed. Isotopic substitution spectra were utilized to obtain precise structural information on the π-π stacking configurations, with their stability further verified via the observation of the helium-doped clusters. Theoretical calculations indicate that dispersion interactions dominate the binding energies of these dimers. The study reveals that due to the small barriers for configurational transitions, only one isomer, either parallel or T-shaped, is likely to persist in the gas phase, unless the two isomers have nearly equal binding energies, as seen in the C₄H₄S-PhF complex.