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 ( A) increases drastically from the antiperiplanar ( ap) to the synperiplanar ( sp) HCCOH conformation. The kinetics of olefin epoxidation with H2O2 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.