Unveiling the Conformations and van der Waals Interactions between Rare Gas and Bicyclic Aromatic: Rotational Spectroscopy of Benzofuran Complexes with Argon and Krypton.

By integrating Fourier transform microwave spectroscopy with quantum chemical calculations, a detailed conformational and structural analysis of the benzofuran (BF) complexes with argon (BF-Ar) and krypton (BF-Kr) is conducted. The conformations of each complex are explored utilizing the Conformer-Rotamer Ensemble Sampling Tool method for conformational searches, with subsequent geometry optimizations using MP2, RI-SCS-MP2, and density functional theory methods. Experimental results corroborate that the most stable conformations for both complexes feature the rare gas (Rg) atom positioned above the BF ring. In particular, Ar and Kr atoms prefer to locate above the C5 atom, with the Rg···C5 line nearly perpendicular to the BF plane. The Ar and Kr atoms are located ≈3.491 and 3.580 Å from the center of mass of BF. The natural bond orbital analysis indicates that the second-order perturbation energy for the Rg···π interaction is about 3.1 and 3.0 kJ mol^-1 in the BF-Ar and BF-Kr complexes, respectively. Symmetry-adapted perturbation theory energy decomposition analysis reveals that dispersion forces are predominant in these complexes. This study provides insights into the nature of van der Waals interactions between bicyclic heteroaromatics and Rg.