PRESSURE-DEPENDENCE OF OH STRETCHING VIBRATIONS .1. FLUOROALKYL ALCOHOLS IN NONPOLAR-SOLVENTS

The OH vibrations of 1H,1H-heptafluorobutyl alcohol and perfluoro-tert-butyl alcohol in perfluoroheptane and poly(chlorotrifluoroethene) have been studied by high-pressure infrared spectroscopy up to 10 GPa. For monomeric OH groups increasing the pressure induces at first a shift to lower wavenumbers up to a characteristic maximum shift and then a subsequent quasi-linear shift to higher wavenumbers. The gas-phase frequency and negative shifts can be obtained at the highest pressures in the condensed phase. The maximum shifts correlate linearly with the system's frequency shifts at ambient pressure. The characteristic pressures, at which the maximum shift and the gas-phase frequency are observed, also increase with the ambient-pressure OH shift. The OH vibration bands broaden with increasing pressure. A pressure-induced splitting due to specific OH-solvent interactions has been observed for perfluoro-tert-butyl alcohol in poly(chlorotrifluoroethene). Attempts are made to explain the high-pressure results by a linear semiclassical model, which yields frequency shifts that depend on the intermolecular distance by additive superposition of the gas-phase OH Morse potentials and OH-solvent Lennard-Jones potentials at different O-solvent distances. It follows from the model that the maximum shifts are due to the maximum force between the OH group and the solvent. The calculations yield OH-solvent potentials, different for each alcohol/solvent system in accordance with the alcohol acidities and the solvent polarizabilities. The ambient-pressure OH frequency shifts correlate linearly with the well depths of the estimated OH-solvent potentials. © 1990 American Chemical Society.