From hydrogen bond donor to acceptor: the effect of ethanol fluorination on the first solvating water molecule

Supersonic jet FTIR spectra of the OH stretching vibrations in complexes of mono-, di- and trifluoroethanol with water are presented. In contrast to the non-fluorinated ethanol case, the fluorinated alcohols are all shown to act as O-H center dot center dot center dot O hydrogen bond donors towards water. This is found to be mostly a consequence of the intramolecular electron-withdrawing effect of the fluorine atoms and, with decreasing importance for increasing fluorination, due to the attractive intermolecular contact between one of the dangling water OH groups and the fluorine atoms. The findings provide a stepwise rationalization for the hydrophobic properties of the pharmaceutically important trifluoromethyl group.