Modulation of π character upon complexation captured by molecular rotation spectra.

Two configurations of the furan-CF 3 Cl complex have been observed by high-resolution rotational spectroscopy. One is characterized by a dominant Cl lone pair⋯π* aromatic interaction and the other is stabilized by a C-Cl···π -CC- halogen bond. This is the first rotational spectroscopic evidence, to the best of our knowledge, that shows how a complexation with a partner like CF 3 Cl (the weak lone pair belt of Cl, to be more specific) can modulate both the aromatic π* and diene π characters of a heteroaromatic molecule in the formation of non-covalent interactions. The results emphasize the partner molecules' role in modulating the π electron structure and will not only deepen our understanding on non-covalent interactions but also lead to better designs of heteroaromatic-based drugs and materials.