On the nature of the water-hexafluorobenzene interaction

The interaction between water and hexafluorobenzene is discussed in light of an nb initio study. It is found that a dimer is formed with a binding energy of about -2 kcal/mol, with a geometry such that the oxygen of water is above the hexafluorobenzene molecule, with both hydrogen atoms pointing out of the ring, and the water C-2 axis is collinear with the main symmetry axis of the aromatic compound. In addition, the intermolecular potential surface (IPS) has been fully characterized. It comes out that the rotational motion of the water molecule around its C-2 axis is nearly "free", whereas the two other bending motions are found to be strongly hindered. These results are discussed in comparison with those reported in the literature for the water-benzene dimer, in which a weak e(pi)-H hydrogen bond interaction has been reported. We argue that in the water-hexafluorobenzene system the oxygen atom acts as a Lewis base and that the aromatic ring plays the role of a Lewis acid, owing to the withdrawing effect of the fluorine atoms on the pi-electronic distribution of the cycle. These results have been used to rationalize the far-infrared experiments that we have previously reported on water diluted in organic solvents. We emphasize that the viewpoint of tho isolated dimer provided by our ab initio study could be transposed to the liquid state if the observation time of the technique is shorter than the lifetime of the dimer. It turns out that mid-infrared spectroscopy is more adapted, than far-infrared absorption, to put in evidence the existence of very labile dimers in the liquid phase, taking into account its shorter time window of observation.