Effect of pressure on molecular conformations. II. T r a n s–g a u c h e equilibrium of 1,2-dichloroethane and 1,2-dibromoethane

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Abstract
The effect of pressure on the trans–gauche equilibrium of 1,2-dichloroethane in n-hexane solution and of 1,2-dibromoethane in 2-methylbutane and acetonitrile solutions has been measured by the Raman spectrum. The relative integrated intensities of the carbon–halogen stretching vibrations of the two conformers has been taken as a measure of their relative concentrations. The volume change at the trans–gauche transformation for 1,2-dichloroethane is −3.8 ± ∼0.2 and −3.5 ± ∼0.2 cm3 mol−1, respectively, in 20% and 30% v/v solution in n-hexane at 21 °C, and for 1,2-dibromoethane it is −4.5 ± ∼0.7 cm3 mol−1 in 10% and 20% v/v solution in 2-methylbutane and −2.0 and −1.8±∼0.7 cm3 mol−1, respectively, in 10% and 20% solutions in acetonitrile at 21 °C. The contributions to the volume change due to the overlapping of the halogen atoms in the gauche conformation and to the electrostatic effects as represented by the interaction of molecules having dipole and quadrupole moments with a dielectric continuum have been calculated. The calculated and experimental volume changes agree reasonably well.