Effects of substituting group on the hydrogen bonding in phenol-H2O complexes:: Ab initio study

Calculations are presented for the ortho- and para-substituted phenol-H2O complexes. A variety of conformers are predicted, and their relative energies are compared. Binding energies of the complexes are computed, and detailed analysis is presented on the effects of substitution on the strength of the hydrogen bonding in the complexes. F- and Cl- (NH2- and OH-) substituted complexes are studied to analyze the effects of electron-withdrawing (electron-donating) groups. In para-substituted complexes, the electrostatic (inductive and resonance) effects influence the binding energies in opposite fashion, depending on whether the hydroxyl group is proton-donating or -accepting. The binding energy of the complex increases (decreases) by the electron-withdrawing substituent when the phenolic OH group is proton-donating (-accepting), and the reverse is true for the electron-donating substituents. For ortho-substituted complexes, direct involvement of the substituting group and the geometry change of the hydrogen bond should be invoked to elucidate the complicated pattern of the binding energy of the complexes. We also suggest that the frequency of the phenolic OH stretching mode of the complex may help elucidate the role of the OH group, determining whether the OH group is proton-donating or -accepting.