Fast Collective Hydrogen-Bond Dynamics in Hexafluoroisopropanol Related to its Chemical Activity

Using fluorinated mono-alcohols, in particular hexafluoro-2-propanol (HFIP), as a solvent can enhance chemical reaction rates in a spectacular manner. Previous work has shown that this enhancement is related to the hydrogen-bond structure of these liquids. Here, we investigate the hydrogen-bond dynamics of HFIP and compare it to the dynamics of its non-fluorinated analog, 2-propanol. Time-resolved and two-dimensional infrared spectroscopy show that the dynamics of single hydrogen-bonds (occurring on a time scale of a few picoseconds) is about twice as slow in HFIP as in 2-propanol. Surprisingly, from dielectric spectroscopy we find the opposite behavior for the collective hydrogen-bond dynamics: the time scale for collective hydrogen-bond rearrangements is 3 times faster in HFIP than in 2-propanol, with a characteristic time scale of 140~ps. This difference in time scales indicates that the hydrogen-bonded clusters in HFIP are smaller than in 2-propanol. The smaller cluster size can contribute to the enhanced reaction rates in HFIP by increasing the concentration of reactive, terminal OH groups of the clusters, whereas the fast collective dynamics can increase the rate of formation of hydrogen bonds between reactants and these terminal OH groups. The longer lifetime of the individual hydrogen bonds in HFIP can enhance the stability of the hydrogen-bonded clusters, and so increase the probability of reactant-solvent hydrogen bonding resulting in a reaction.