Hydrogen Bonding in Low-Temperature Matrices: 1. Proton Donor Abilities of Fluoroalcohols. Comparative Infrared Studies of ROH...O(CH3)2 Complex Formation in the Gas Phase, in CCl4 Solution, and in Solid Argon

Article abstract of DOI:10.1021/j150663a029

The proton donor-acceptor complexes formed by a series of fluoroalcohols with systematically increasing acidity (pK_a's from 14.5 to 5.4) have been studied with dimethyl ether (DME) as a reference base in argon matrices, in CCl4 solutions, and in the gas phase.The intermolecular bond formation O-H...O is discussed on the basis of changes of the O-H stretching absorptions in the infrared spectra.A stepwise increase of the proton donor abilities of the fluoroalcohols is achieved by successively introducing fluorine in β-position of the alcohols as indicated by the increasing frequency shift of the O-H stretching vibrations in the O-H...O complexes.The observed frequency shifts Δν increases as one passes from the gas phase to CCl4 solutions and argon matrices.Data for ΔH⁰, ΔS⁰, and ΔG⁰ have been derived from the solution measurements on the basis of the temperature-dependent intensity changes of the "free" O-H stretching bands.A comparison of the O-H stretching frequencies of the alcohol monomers obtained from the three phases has also been made.The small shifts observed in argon matrices on the order of 19 +/- 4 cm^-1 are characteristic for weak interactions of the van der Waals type.The solvent shift in CCl4 is about twice as high and was found to be on the order of 45 +/- 8 cm^-1, indicating a much stronger interaction with the solute molecules.It is concluded that it is advantageous, where possible, to study a particular hydrogen-bonded system in different phases in order to get a deeper insight into intermolecular interactions ranging from very weak to very strong and to distinguish hydrogen bond interaction in condensed phases from other intermolecular forces or environmental effects.