text: The para form whose lowest level is J = 0 is more stable by 1.455 kJ/mol than the ortho form whose lowest level is J = 1. The ratio between numbers of ortho and para molecules is about 3:1 at standard temperature where many rotational energy levels are populated, favoring the ortho form as a result of thermal energy. However, at low temperatures only the J = 0 level is appreciably populated, so that the para form dominates at low temperatures (approximately 99.8% at 20 K). The heat of vaporization is only 0.904 kJ/mol. As a result, ortho liquid hydrogen equilibrating to the para form releases enough energy to cause significant loss by boiling.
  Thermal properties
  Applying the rigid rotor approximation, the energies and degeneracies of the rotational states are given by:
  .
  The rotational partition function is conventionally written as:
  .
  However, as long as the two spin isomers are not in equilibrium, it is more useful to write separate partition functions for each:
  The factor of 3 in the partition function for orthohydrogen accounts for the spin degeneracy associated with the +1 spin state; when equilibrium between the spin isomers is possible, then a general partition function incorporating this degeneracy difference can be written as:
  The molar rotational energies and heat capacities are derived for any of these cases from: