CAN CONFORMATIONAL EQUILIBRIA BE TUNED IN SUPERCRITICAL-FLUID SOLUTION - AN IR SPECTROSCOPIC STUDY OF TRANS/GAUCHE ISOMERISM OF HEXAFLUOROPROPAN-2-OL IN SUPERCRITICAL SF6 AND CHF3 SOLUTIONS

We report the effect of fluid density and polarity on the trans reversible arrow gauche equilibrium of the rotamers of (CF3)(2)CHOH in supercritical SF6 (scSF(6)) and CHF3 (scCHF(3)) solutions. The equilibrium is followed by IR spectroscopy in the nu(O-H) region (3700-3500 cm(-1)). An unusual aspect is the use of long and short path length IR cells in tandem to circumvent problems of IR absorption by the fluids. A comparison of the effects of increasing pressure in the nonpolar fluid scSF(6) and the polar fluid scCHF(3) allows us to separate the effect of increasing density, which happens to be small, and the effect of increasing dielectric constant, which favors the gauche rotamer. The change in equilibrium constant for trans = gauche is correlated to the polarity of the fluid via the Kirkwood parameter.