Mid-IR absorption spectroscopy has been used to characterize the solvent effect on the OH stretching vibrations ?OH of several photoacids and ethanol. We distinguish the dielectric (non-specific) effect of the solvent on the observed red-shifts in ?OH from the much larger red-shift caused by hydrogen (H)-bonding interactions with the solvents. To isolate the solvent dielectric constant e effect on ?OH, the stretch vibration was measured in neat solvents and analyzed by the Kamlet-Taft solvent polarity parameters. We observed that for hydrogen-bonding complexes with invariant base, DMSO, ?OH tends to vary inversely with, e exhibiting different slopes for solvents comprising molecules with and without a permanent dipole moment. We find that ??OH depends importantly on the difference between the proton affinities of the conjugate base of the photoacid and the proton acceptor. A novel two-state Valence-Bond-based theory indicates that our experimental observations primarily arise from the OH oscillator acquiring a more ionic-like character in the vibrational excited-state. This results in a stronger H-bond and in a more anharmonic potential for the OH vibration.

Mid-IR absorption spectroscopy of hydrogen-bonded complexes in ground and excited electronic states of photoacids

248th National Meeting (2014)

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