On the origin of donor O–H bond weakening in phenol-water complexes

Abstract

Matrix isolation infrared spectroscopy has been used to investigate intermolecular interactions in a series of binary O–H⋯O hydrogen bonded phenol-water complexes where water is the common acceptor. The interaction at the binding site has been tuned by incorporating multiple fluorine substitutions at different aromatic ring sites of the phenol moiety. The spectral effects for the aforesaid chemical changes are manifested in the infrared spectra of the complexes as systematic increase in spectral shift of the phenolic O–H stretching fundamental (Δν_O–H). While ν_O–H bands of the monomers of all the fluorophenols appear within a very narrow frequency range, the increase in Δν_O–H of the complexes from phenol to pentafluorophenol is very large, nearly 90%. The observed values of Δν_O–H do not show a linear correlation with the total binding energies (ΔE_b) of the complexes, expected according to Badger-Bauer rule. However, in the same Δν_O–H vs ΔEb plot, nice linear correlations are revealed if the complexes of ortho-fluorophenols are treated separately from their meta/para-substituted analogues. The observations imply that in spite of having the same binding site (O–H⋯O) and the same chemical identities (phenolic), the complexes of ortho and non-ortho fluorophenols do not belong, from the viewpoint of detailed molecular interactions, to a homologous series. Linear correlations of Δν_O–H are, however, observed with respect to the electrostatic component of ΔE_b as well as the quantum mechanical charge transferinteractionenergy (ECT). From quantitative viewpoint, the latter correlation along with the associated electronic structure parameters appears more satisfactory. It has also been noted that the observed Δν_O–H values of the complexes display a linear relationship with the aqueous phase pKa values of the respective phenol derivatives.