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

摘要

The effect of pressure on the infrared spectrum of 1,2hyphen;dichloroethane in 2hyphen;methylbutane solution has been measured up to 36 kbar at 22thinsp;deg;C using a diamond anvil cell. The sample pressure was measured by the frequency shift of the 764 cmminus;1band of 2hyphen;methylbutane, which was itself calibrated by the frequency shift of the ngr;3vibration of the nitrite ion dissolved in sodium bromide. The effect of pressure on the internal rotation angle of thegaucheform has been determined from the relative intensity of the two carbonndash;chlorine stretching vibrations by applying a simple model. It decreases at the rate 2.7deg; kbarminus;1in the range 0ndash;5 kbar, which is close to the 2.6deg; kbarminus;1that was obtained by applying the same model to the published relative Raman intensity of the two Cndash;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 5ndash;30 kbar a decrease in the internal rotation angle causes the Clndash;Cndash;C angles to open, because of the chlorinendash;chlorine repulsion, by enough to keep the angle between the Cndash;Cl bonds approximately constant. The volume change at thetransndash;gauchetransformation has been determined as minus;2.9plusmn;0.4 cmminus;3thinsp;molminus;1from the relative integrated intensities of thegaucheCH2rocking andtransCH2wagging 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.