Quantal Factors in the Joule-Thomson cooling of Hydrogen and Deuterium

This paper presents a theoretical analysis of the Joule-Thomson cooling of normal hydrogen and deuterium from an initial temperature of 64 �K. Below 70 atm the cooling is greater for H2 than for D2, but at higher pressures it is less. It is found that the difference at low pressures is due to the lower heat capacity of H2, associated with the more quantized rotation of its molecules. The difference at high pressures is due to the less negative enthalpy of imperfection of H2, and this in turn can be attributed to the higher zero-point energy for translational motion in the lighter isotope. Both differences can be accounted for quantitatively.