Switching Aromatic Character by Complexation: π to π* Change Seen in Molecular Rotation Spectra

A dominating F···π*_aromatic interaction is found to govern the benzaldehyde···tetrafluoromethane complex, as revealed by this rotational spectroscopic investigation. Secondary F···π*_-C=O- and C σ*_CF4···π_aromatic interactions also contribute to the stability of the observed isomer. Narrow splittings have been observed in the rotational spectrum originating from a 3-fold internal rotation of CF₄ above the aromatic moiety, and a corresponding V₃ barrier was determined to be 1.572(14) kJ mol^-1. This is the first rotational spectroscopic evidence in the literature implying that the aromatic π* antibonding orbital can be activated not only by electron-withdrawing substituents but also by complexation partners containing atoms with high electronegativity, like CF₄. The results emphasize the partner molecules' role to modulate the π electron structure and show a change in the orbital character (π or π*) when participating in the formation of noncovalent interactions.

Dynamic chiral self-recognition in aromatic dimers of styrene oxide revealed by rotational spectroscopy

Chiral molecular recognition is a pivotal phenomenon in biomolecular science, governed by subtle balances of intermolecular forces that are difficult to quantify. Non-covalent interactions involving aromatic moieties are particularly important in this realm, as recurring motifs in biomolecular aggregation. In this work, we use high-resolution broadband rotational spectroscopy to probe the dynamic conformational landscape enclosing the self-pairing topologies of styrene oxide, a chiral aromatic system. We reach a definite assignment of four homochiral and two heterochiral dimers using auxiliary quantum chemistry calculations as well as structure-solving methods based on experimental isotopic information. A complete picture of the dimer conformational space is obtained, and plausible routes for conformational relaxation are derived. Molecular structures are discussed in terms of conformational flexibility, the concerted effort of weak intermolecular interactions, and their role in the expression of the molecular fit.