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···πaromatic 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.