VAPOR-PRESSURE ISOTOPE EFFECTS IN LIQUID-HYDROGEN CHLORIDE

The difference between the vapour pressures of HCl and DCl has been measured over the temperature range 170-203 K by a differential manometric technique in a precision cryostat. In this range the vapour pressure of HCl is higher than that of DCI by 3.2% at 170 K, decreasing to 0.9% at 200 K. The reduced partition function ratios f(l)/f(g) derived from the vapour pressure data can be described by the equation ln(f(l)/f(g)) = (3914.57+/-10)/T2 - (17.730+/-0.055)/T. The experimentally observed H-D vapour pressure isotope effect, together with the values on the Cl-35-Cl-37 isotope effect available in the literature, is interpreted in the light of the statistical theory of isotope effects in condensed systems by using spectroscopic data of the vapour and liquid phases. The results indicate that the rotation in liquid hydrogen chloride is hindered. Temperature-dependent force constants for the hindered translational and rotational motions were invoked in order to obtain better agreement between the model calculation and experiment.