Effect of pressure on molecular conformations. V. The internal rotation angle of 1,2-dichloroethane by infrared spectroscopy

The effect of pressure on the infrared spectrum of 1,2-dichloroethane in 2-methylbutane solution has been measured up to 36 kbar at 22 °C using a diamond anvil cell. The sample pressure was measured by the frequency shift of the 764 cm−1 band of 2-methylbutane, which was itself calibrated by the frequency shift of the ν3 vibration of the nitrite ion dissolved in sodium bromide. The effect of pressure on the internal rotation angle of the gauche form has been determined from the relative intensity of the two carbon–chlorine stretching vibrations by applying a simple model. It decreases at the rate 2.7° kbar−1 in the range 0–5 kbar, which is close to the 2.6° kbar−1 that was obtained by applying the same model to the published relative Raman intensity of the two C–Cl stretching vibrations. Above 5 kbar, the ratio of the intensities of the two vibrations is constant up to 30 kbar within the experimental uncertainty. This suggests that in the range 5–30 kbar a decrease in the internal rotation angle causes the Cl–C–C angles to open, because of the chlorine–chlorine repulsion, by enough to keep the angle between the C–Cl bonds approximately constant. The volume change at the trans–gauche transformation has been determined as −2.9±0.4 cm−3 mol−1 from the relative integrated intensities of the gauche CH2 rocking and trans CH2 wagging vibrations as a function of pressure. The value obtained agrees within experimental errors with the value which was obtained in a similar way from earlier Raman spectra.