On the nature of the water-hexafluorobenzene interaction

The interaction between water and hexafluorobenzene is discussed in light o f an nb initio study. It is found that a dimer is formed with a binding ene rgy 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 ax is of the aromatic compound. In addition, the intermolecular potential surf ace (IPS) has been fully characterized. It comes out that the rotational mo tion of the water molecule around its C-2 axis is nearly "free", whereas th e two other bending motions are found to be strongly hindered. These result s 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 oxy gen 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 ra tionalize the far-infrared experiments that we have previously reported on water diluted in organic solvents. We emphasize that the viewpoint of tho i solated dimer provided by our ab initio study could be transposed to the li quid state if the observation time of the technique is shorter than the lif etime of the dimer. It turns out that mid-infrared spectroscopy is more ada pted, than far-infrared absorption, to put in evidence the existence of ver y labile dimers in the liquid phase, taking into account its shorter time w indow of observation.