Solvent induced red-shifts in the IR absorption of the O-H stretch vibration of para substituted phenols hydrogen-bonded to various bases*

Infrared absorption spectroscopy was  used to characterize the OH stretching vibrations of the family of paraphenols derivatives  in non-polar and polar solvents. The hydrogen bond interaction between the substituted phenols (X = OCH₃, CH₃, H,F,  Cl, Br, I,CN, NO₂) and DMSO and acetonitrile   were investigated in relation to the dielectric constant of the solvent ε and the differences of the proton affinities ΔPA of the conjugate base of the paraphenols and the complexed base.  For a given H-bonded complex, we find  that νOH tends to vary inversely with ε, exhibiting different slopes for polar and nonpolar solvents, i.e., different slopes for solvents comprising molecules with and without a permanent dipole moment, respectively. We use a two-state valence-bond-based theory to analyze our experimental data. This demonstrates that the OH oscillator acquires a more ionic-like character in the vibrational excited state, i.e., gaining charge transfer; this results in a stronger H-bond in a more anharmonic potential for the OH vibration. Our experimental and calculated  ν_OH  vs. 1/ε plots    provide a way to clearly distinguish between polar and non-polar  solvents by their different  effect  on contact 1:1 hydrogen-bonding complexes of paraphenols and  a  base.