Raman study of the trans-gauche conformational equilibrium of 1,2-dichloroethane in water:: Experimental evidence for the hydrophobic effect

The effect of water on the trans-gauche conformational equilibrium of 1,2-dichloroethane has been studied by Raman spectroscopy. From the intensities of C-Cl stretching bands and their temperature dependencies, we determined the free energy and enthalpy difference between the conformers in water and organic solvents. Although the solvent effect on the free energy apparently agrees with a dielectric continuum model, the enthalpy in aqueous solutions deviates largely from the values expected from that model. The positive deviations (similar to 3.5 kJ/mol) in the enthalpy are compensated by an increase in the entropy term. This behavior is consistent with the picture of a hydrophobic interaction. (C) 1999 American Institute of Physics.