Global quantitation of solvent effects on the isomerization thermodynamics of 1,2-dichloroethane and trans-1,2-dichlorocyclohexane

The conformational equilibrium of the trans-gauche isomerization of 1,2-dichloroethane (DCE) and the axial, axial-equatorial, equatorial isomerization of trans-1,2-dichlorocyclohexane (T12D), both dissolved in diethyl ether, are studied using Raman spectroscopy. The C-Cl stretch band areas of the two conformers are measured as a function of temperature and pressure. The resulting area ratios, calibrated against NMR measurements, are used to globally quantitate the complete set of isomerization thermodynamic functions in solution, as well as the corresponding solvent excess contributions (by comparison with vapor-phase results). The effects of pressure on the two isomerization processes are found to be opposite in sign, as the polar isomer (gauche) of DCE is favored at high pressure and low temperature, while the more polar (equatorial, equatorial) form of T12D is driven in the opposite direction. The excess enthalpy (and energy) of reaction is negative for both reactions, while the reaction volume is negative for the DCE and positive for T12D. The results reveal the different impact of attractive (cohesive) and repulsive (cavity formation) perturbations on various thermodynamic functions.