Unveiling the "booster effect" of fluorinated alcohol solvents: aggregation-induced conformational changes and cooperatively enhanced H-bonding.

The influence of conformation and aggregation on the hydrogen bond donor ability of fluorinated alcohol solvents [1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1-phenyl-2,2,2-trifluoroethanol (PhTFE)] was explored theoretically (DFT) and experimentally (NMR, kinetics, crystal structure analyses). The detailed DFT analysis revealed a pronounced dependence of the H-bond donor ability on the conformation along the CO-bond of the monomeric alcohols. The donor orbital energy (sigma*(OH)) decreases and the molecular dipole moment (mu) increases drastically from the antiperiplanar (ap) to the synperiplanar (sp) H(C)COH conformation. The kinetics of olefin epoxidation with H(2)O(2) in HFIP indicate higher order solvent aggregates (2-3 monomers) to be responsible for the activation of the oxidant. Single-crystal X-ray analyses of HFIP and PhTFE confirmed the existence of H-bonded aggregates (infinite helices, ribbons, and cyclic oligomers) and the predominance of sc to sp conformations of the fluoroalcohol monomers. These aggregate structures served as the basis for a DFT analysis of the H-bond donor ability at the terminal hydroxyl group of HFIP mono- to pentamers. Both the LUMO energy and the natural charge of the terminal hydroxyl proton indicated a substantial cooperative influence of dimerization and trimerization on the H-bond donor ability. We therefore conclude that dimers and trimers, with the individual monomers in their sc to sp conformation, play a crucial role for the solvolytic and catalytic effects exerted by HFIP, rather than monomers.