Dual role of π electrons in furan mapped on the furan–CF3Cl complex studied by rotational spectroscopy combined with theoretical calculations
The very recent rotational study on CF3Cl–CO2 reveals that the Cl “equatorial belt” in CF3Cl can be activated by the π-hole region on CO2, forming a C∙∙∙Cl tetrel bond. On the other hand, the aromatic π* antibonding orbital can be activated by the partner with high electronegativity, like CF4, as found in the rotational study of the benzaldehyde–CF4 complex. In this work, through the combination of rotational spectroscopy and quantum chemical calculations, we studied the structures and noncovalent interactions between furan and CF3Cl. Two isomers were observed in the pulsed jet. For the most stable one, the Cl “equatorial belt” in CF3Cl prefers to link with the delocalized aromatic π* antibonding orbital in furan. The second observed isomer, instead, is stabilized by the Cl∙∙∙π halogen bond formed between the Cl σ-hole of CF3Cl and C=C-C=C diebe structure of furan. To support experiment, calculations beyond the harmonic-oscillator approximation were carried out, which allow the inclusion of vibrational averages in the rotational constants. Thanks to the availability of high-resolution reference experimental data for the observed isomers, a detailed study of the quality of electronic structure calculations based on DFT functionals and the treatment of large amplitude motions related to the intermolecular vibrations in complexes was carried out. This work has paved the way into devising better control mechanisms to reduce the risk of error compensations and identify possible pitfalls in anharmonic calculations based on perturbed approaches, and helped design more reliable computational protocols to support the experimental analysis for new and floppy systems. In particular, the validity of hybrid schemes combining harmonic force field and structural parameters at a higher level of theory with anharmonic constants built at a lower level has been assessed.
 Yang Zheng, Sven Herbers, Qian Gou, Walther Caminati, Jens-Uwe Grabow, The Journal of Physical Chemistry Letters, 2021, 12, 3907–3913.
 Hao Wang, Junhua Chen, Chunguo Duan, Xuefang Xu, Yang Zheng, Jens-Uwe Grabow, Qian Gou, Walther Caminati, The Journal of Physical Chemistry Letters, 2021, 12, 5150–5155.