P, T DEPENDENCE OF SELF-DIFFUSION IN 2-FLUOROETHANOL, 2,2-DIFLUOROETHANOL AND 2,2,2-TRIFLUOROETHANOL

The self-diffusion of ethanols substituted by fluorine at the methyl c arbon has been studied by FTNMR using pulsed gradient spin-echo techni ques. There is much interest in fluoroethanols, particularly trifluoro ethanol (TFE), because of its use in inducing conformational change in peptides and proteins. One of the rationales for looking at the fluor oethanols was to study the effect of changing hydrogen bond structures by fluorine substitution upon the self-diffusion behaviour. One impor tant property of TFE in this context is its ability to form intramolec ular hydrogen bonds. The results for the diffusion measurements showed that the effects of adding fluorine appear to be steric in nature, wi th little effect of changing the hydroxyl hydrogen bond strength. 2-fl uoroethanol has selfdiffusion coefficients very similar in magnitude a nd p, T dependence to 1-propanol. These two molecules have similar mas ses. 2,2-difluoroethanol has a more pronounced temperature dependence than 2-fluoroethanol or 2,2,2-trifluoroethanol. In addition, analysis of the experimental data to a VTF type equation reveals that 2,2-diflu oroethanol has a much higher ideal glass transition temperature, T-0, than either 2-fluoroethanol or TFE. These data indicate that steric co nformations in difluoroethanol produce more stable hydrogen bonded str uctures than in the other two liquids. There is little evidence from t hese self-diffusion data of any effect of intra-molecular hydrogen bon ding. This is thought to occur, particularly for 2,2,2-trifluoroethano l. (C) 1998 Elsevier Science B.V.