Conformational Studies by Vibrational Spectroscopy under High Pressure

Abstract

Liquids with freezing points in the range 180–290 K at atmospheric pressure typically crystallize under pressures of 2–60 kbar at roan temperature, a region easily accessible in a diamond anvil cell (DAC). High pressure crystallization can be useful for conformational studies when low temperature crystallization cannot be achieved or when the high pressure crystal contains a different conformer from the low temperature crystal.

In most cases the same conformer is present in the low temperature and in the high pressure crystals and this seems invariably to be the case when the ΔH value between the conformers is larger than 4 kJ mol^-1. However, high pressure will favor the conformer with the smallest molar volume as demonstrated for 1,1,2-trichloroethane as well as in a number of halogenated cyclohexanes for which the axial conformers had smaller volumes than those of the equatorial ones. Reversible phase transitions in bicyclohexyl involving at least two of the six possible conformers were observed under variable pressure and temperature conditions.

The conformational equilibria in the neat liquid or in solution as a function of pressure have been studied for certain chain like and cyclic molecules. The difference in partial molar volumes can be determined from these data and a number of these results have been reviewed.