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

A dominating F center dot center dot center dot p*(aromatic) interaction is found to govern the benzaldehyde center dot center dot center dot tetrafluoromethane complex, as revealed by this rotational spectroscopic investigation. Secondary F center dot center dot center dot pi*(- C=O-) and C sigma* (CF4)center dot center dot center dot pi(paromatic) 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 V-3 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 pi* 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 p electron structure and show a change in the orbital character (pi or pi*) when participating in the formation of noncovalent interactions.