Monitoring intermolecular forces by high pressure infrared spectroscopy?

Abstract

Abstract A semiclassical linear model is discussed, which is able to describe the appearance of a maximum frequency shift and zero shift in condensed phase, which have been experimentally determined before in high pressure IR spectroscopic investigations of the OH stretching vibration of fluoro-alcohols in non-polar solvents. The model approximates the effective OH vibrational potential in the solution by the sum of the gasphase OH potential and a perturbation potential in Lennard-Jones form. To model the pressure dependence the two potentials were shifted with respect to each other. The interpretation of the pressure induced frequency shifts by this model yields some general relations. The maximum red shift is linearly related to the ratio of the well depth to the minimum distance. For Lennard-Jones-potentials a linear relation exists between the maximum frequency shift and the gradient at the inflection point of the Lennard-Jones-potential, which gives the maximum attractive intermolecular force. The intermolecular distance at which the maximum shift occurs is independent of the well depth and only depends on the minimum distance of the perturbation potential. This also valid for the distance at which the zero shift occurs. At pressures less than 10 GPa the frequency shift of the OH group of fluoro-alcohols in non-polar solvents seems to be a linear measure of the gradient of the intermolecular potential, that is of the intermolecular force. To find the simple linear function between frequency shift and intermolecular force requires investigation of the perturbance at a distance, which is distinctly larger than the bond length of the OH group. The interaction potentials for five binary fluoro-alcohol/non-polar solvent systems are estimated.