Correlation of ν_OH spectral shifts of phenol–benzene O–H···π hydrogen-bonded complexes with donor’s acidity: a combined matrix isolation, infrared spectroscopy, and quantum chemistry study

Abstract

O–H stretching infrared fundamentals (ν_OH) of phenol and a series of fluorophenol monomers and their 1:1 complexes with benzene have been measured under a matrix isolation condition (8 K). Spectral analysis reveals that ring fluorine substitutions have little effect on phenolic ν_OH as long as the molecules in the matrix are fully dispersed as monomers. The substitution effects are pronouncedly manifested only when the phenols are complexed with benzene, and the measured shift in phenolic νOH from the monomer value varies from ∼78 cm^–1 in phenol to ∼98 cm^–1 in 3,4,5-trifluorophenol. The spectral shifts are found to display a linear correlation with the aqueous phase acid dissociation constants (pK_a) of the phenols. The spectral changes predicted by electronic structure calculations at several levels of theory are found to be consistent with the observations. Such correlations are also found to exist with respect to different energetic, geometric, and other electronic structure parameters of the complexes. Atoms in Molecules (AIM) analysis shows a distinct bond critical point due to accumulation of electron density at the hydrogen-bonding site. The variation of electron densities both on the hydrogen bond as well the donor O–H group is in accordance with the experimentally observed νO–H of the various fluorophenol–benzene complexes. Partitioning of binding energies into components following the Morokuma–Kitaura scheme shows that the π-hydrogen-bonded complexes are stabilized predominantly by dispersion interactions, although electrostatics, polarization, and charge-transfer terms have appreciable contribution to overall binding energies. NBO analysis reveals that hyperconjugative charge-transfers from the filled π-orbitals of the hydrogen bond acceptor (benzene) to the antibonding σ*(O–H) orbital of the donors (phenols) display correlations which are fully consistent with the observed variations of spectral shifts. The analysis also shows that the O–H bond dipole moments of all the phenolic species are nearly the same, implying that local electrostatics has only a little effect at the site of hydrogen bonding.