Hydrophobic Hydration of Fluoroalkyl (C-F) is Distinctly Different from That of Its Hydrogenated Counterpart (C-H), as Observed by Raman Difference with Simultaneous Curve Fitting Analysis

Interaction of fluoroalkyl with water is the key parameter tailoring the applications of organofluorines in diverse fields. Nevertheless, how fluorination modifies an alkyl group's interaction with water and the associated local hydrophobicity is largely unknown. Using Raman difference spectroscopy with simultaneous curve fitting (RD-SCF) and MD simulation, we show that fluorination breaks down the tetrahedral water structure that is otherwise present around the alkyl group (hydrophobic hydration). Electrostatic perturbation (due to fluorination) restricts the water molecules to adopt the symmetrically hydrogen-bonded tetrahedral structure. As a result, the relative population of strongly H-bonded water decreases, while that of the weakly interacting dangling OH increases in the hydration shell of a fluoroalkyl group. These structural changes of water make the accommodation of fluoroalkyl energetically costly compared to that of its hydrogenated counterpart, which is manifested as enhanced hydrophobicity for the former.