Probing the Lone Pair⋯π-Hole Interaction in Perfluorinated Heteroaromatic Rings: The Rotational Spectrum of Pentafluoropyridine·Water

The rotational spectrum of the weakly bound complex pentafluoropyridine·water has been investigated with pulsed jet Fourier transform microwave spectroscopy. From the analysis of the rotational parameters of the parent species and of three water isotopologues, the structural arrangement of the adduct has been unambiguously established. The results show that the full ring fluorination of pyridine has a dramatic effect on its binding properties: It inverts the electron density distribution above the ring, creating a π-hole, with respect to the typical π-cloud of benzene and pyridine. In the complex the water moiety lies above the aromatic ring with the oxygen lone pairs pointing toward its center. This lone pair⋯π-hole interaction stabilizes the adduct, and it is more stable than the in-plane O-H⋯N hydrogen bond normally found in the complexes involving nitrogen heterocyclic aromatic rings. Evidence of a large amplitude motion involving the weakly bound water molecule has also been observed and discussed.