Effects of fluorine substitution on hydrogen bond interactions

The effect of fluorine substitution on a series of hydrogen-bonded (HB) systems has been studied with the aid of theoretical ab initio methods. The effect on HB donors has been examined using a series of fluoromethane derivatives as hydrogen bond donors in their interaction with water. The results indicate a 0.1 Angstrom shortening of the HB bond distance for each additional fluorine atom and 1 kcal/mol increase for the interaction energy. The fluorine effects have been considered in several HB accepters like hexafluorobenzene and a series of small molecules. The pi-cloud of hexafluorobenzene forms stable complexes with electron rich atoms, behaviour that is the opposite to what is observed for benzene. For the small molecules acting as HB accepters, a weakening of the HB is observed. In some cases, the effect is so important that HB properties are reverted as the case of the hexafluorobenzene. Finally, the atomic properties of a series of charge transfer and HB complexes have been compared. (C) 2000 Elsevier Science S.A. All rights reserved.