Cooperative effect on phenolic ν frequencies in 1:1 hydrogen bonded complexes of o-fluorophenols with water: A matrix isolation infrared spectroscopic study.

Matrix isolation infrared spectra of 1:1 complexes of two ortho-fluorophenols, 2-fluorophenol (2-FPh) and 2,6-difluorophenol (2,6-DFPh), with water and benzene have been analyzed in combination with electronic structure calculations to investigate cooperative effect in O-H···O-H···F hydrogen bonded linkage, which manifests as large spectral shifts of the phenolic O-H stretching fundamental. Calculation predicts that a nearly planar cyclic geometry is preferred by the binary water complexes of the syn conformer of 2-FPh as well as 2,6-DFPh, and the observed spectral shifts are in good agreement with the predicted shifts for such conformers. On the other hand, for other possible isomeric structures, the molecular plane of water moiety is oriented perpendicular to that of the fluorophenols, and the observed as well as predicted shifts are smaller than those of the ortho substituted fluorophenols, although the total binding energies are predicted to be larger for the former. The observed spectral shifts are however consistent with local interaction energy parameters, like hyperconjugative charge transfer and accumulation of electron density (ρ) along the O-H···O hydrogen bond path. For the binary O-H···π hydrogen bonded benzene complexes of the fluorophenols, where cooperative interaction is not possible, the observed shifts are consistent with the conformers preferred according to total binding energies as well as local charge transfer effects of the complexes.