Effect of pressure on molecular conformations. III. Internal rotation angle in g a u c h e-1,2-dichloroethane and g a u c h e-1,2-dibromoethane

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Abstract
It is shown that the internal rotation angle in gauche-1,2-dichloroethane and gauche-1,2-dibromoethane can be determined from the ratio of the integrated anisotropic Raman, or integrated infrared, intensities of the two carbon–halogen stretching vibrations. This theory has been used to determine, from the Raman spectrum, the value of the angles of 1,2-dichloroethane in solution in n-hexane and of 1,2-dibromoethane in solution in 2-methylbutane and acetonitrile, and how they vary with pressure. For 1,2-dichloroethane, the angle decreases at the rate of ∼2° kbar−1, and so the cis conformation should become stable at ∼15 kbar. For 1,2-dibromoethane, overlapping bands restrict the accuracy, but the spectrum is consistent with about the same rate of decrease. For trans- 1,2-dibromoethane, the asymmetric carbon–bromine stretching vibration has a measurable Raman intensity, which can only occur if the molecule is distorted from the exact trans conformation. If all the distortion is attributed to the departure of the internal rotation angle from the ideal value, the root-mean-square deviation is ∼10° and ∼15° in 2-methylbutane and in acetonitrile solution, respectively. The mean-square torque causing the distortion is ∼1.5 and ∼3.4 × 10−3 mdyn2 Å2, respectively. About 1.5 × 10−3 mdyn2 Å2 is presumably due to repulsive forces in both solutions, and ∼1.9 × 10−3 mdyn2 Å2 is due to the electrostatic field of the acetonitrile molecules. The rms electrostatic field at the center of a dibromoethane molecule is ∼0.1 V Å−1. A simple model assuming randomly oriented spherical dipolar molecules predicts a field of about one third of this value.