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 carbon has been studied by FTNMR using pulsed gradient spin-echo techniques. There is much interest in fluoroethanols, particularly trifluoroethanol (TFE), because of its use in inducing conformational change in peptides and proteins. One of the rationales for looking at the fluoroethanols was to study the effect of changing hydrogen bond structures by fluorine substitution upon the self-diffusion behaviour. One important property of TFE in this context is its ability to form intramolecular hydrogen bonds. The results for the diffusion measurements showed that the effects of adding fluorine appear to be steric in nature, with little effect of changing the hydroxyl hydrogen bond strength. 2-fluoroethanol has self diffusion coefficients very similar in magnitude and p, T dependence to I-propanol. These two molecules have similar masses. 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-difluoroethanol has a much higher ideal glass transition temperature, To, than either 2-fluoroethanol or TFE. These data indicate that steric conformations in difluoroethanol produce more stable hydrogen bonded structures than in the other two liquids. There is little evidence from these self-diffusion data of any effect of intra-molecular hydrogen bonding. This is thought to occur, particularly for 2,2,2-trifluoroethanol.