title: Effect of Solvent Dielectric Constant and Acidity on the OH Vibration Frequency in Hydrogen-Bonded Complexes of Fluorinated Ethanols.
abstract: Infrared spectroscopy measurements were used to characterize the OH stretching vibrations in a series of similarly structured fluoroethanols, RCH2OH (R = CH3, CH2F, CHF2, CF3), a series which exhibits a systematic increase in the molecule acidity with increasing number of F atoms. This study, which expands our earlier efforts, was carried out in non-hydrogen-bonding solvents comprising molecules with and without a permanent dipole moment, with the former solvents being classified as polar solvents and the latter designated as nonpolar. The hydrogen bond interaction in donor-acceptor complexes formed in solution between the fluorinated ethanol H-donors and the H-acceptor base DMSO was investigated in relation to the solvent dielectric and to the differences ΔPA of the gas phase proton affinities (PAs) of the conjugate base of the fluorinated alcohols and DMSO. We have observed that νOH decreases as the acidity of the alcohol increases (ΔPA decreases) and that νOH varies inversely with ε, exhibiting different slopes for nonpolar and polar solvents. These 1/ε slopes tend to vary linearly with ΔPA, increasing with increasing acidity. These experimental findings, including the ΔPA trends, are described with our recently published two-state Valence Bond-based theory for acid-base H-bonded complexes. Lastly, the correlation of the alcohol's conjugate base PAs with Taft σ* values of the fluorinated ethyl groups CH(n)F(3-n)CH2- provides a connection of the inductive effects for these groups with the acidity parameter ΔPA associated with the H-bonded complexes.
authors.affiliation: †Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel.
authors.forename: Dina
authors.initials: D
authors.lastname: Pines
authors.affiliation: †Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel.
authors.forename: Sharon
authors.initials: S
authors.lastname: Keinan
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Kiefer
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States.|§Chemistry Department, École Normale Supérieure, UMR ENS-CNRS-UPMC 8640, 24 rue Lhomond, 75005 Paris, France.
authors.forename: James T
authors.initials: JT
authors.lastname: Hynes
authors.affiliation: †Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel.
authors.forename: Ehud
authors.initials: E
authors.lastname: Pines
mesh_terms: D004121:Dimethyl Sulfoxide / Q000737:chemistry; D000431:Ethanol / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D010100:Oxygen / Q000737:chemistry; D011522:Protons; D012996:Solutions; D012997:Solvents / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D014732:Vibration
chemical_list: D011522:Protons; D012996:Solutions; D012997:Solvents; D000431:Ethanol; D006859:Hydrogen; D010100:Oxygen; D004121:Dimethyl Sulfoxide