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.