Utilizing a valence bond (VB) model for hydrogen (H-) bonded complexes of the acid dissociation type, i.e. AH�B ? A-�HB+ (but without complete proton transfer), a theory is presented for the shift in proton stretch vibrational frequency ?AH of the H-bonded complex with the solvent's dielectric constant e. Experimental data from several OHO H-bonded complexes are analyzed with this theoretical model. ?AH decreases with increasing e, as the limiting ionic H-bonded complex A-�HB+ becomes less disfavored with respect to the neutral complex AH�B, due to an increased solvation free energy of the ionic complex relative to that of the neutral complex. In this VB perspective, the ?AH magnitude and its variation with e is determined by three characteristics of the H-bonded complex: (i) a structure factor that is a measure of the complex's size, (ii) the equilibrium ground vibrational state dipole moment, and (iii) the vacuum proton affinity difference between the conjugate base of A and the base B.

Valence bond theory for the variation of the proton stretch vibrational frequency in a hydrogen-bonded complex with solvent polarity

245th National Meeting (2013)

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