ortho-hydrogen

Ortho-hydrogen is a form of molecular hydrogen where the two hydrogen nuclei spin in the same direction, known as parallel nuclear spin. Ordinary hydrogen gas is a mixture of ortho-hydrogen and para-hydrogen (where nuclei spin in opposite directions), typically in a three-to-one ratio of ortho- to para-hydrogen.1

These two forms, ortho- and para-hydrogen, differ due to the magnetic interactions of their protons based on their spinning motions. While they are distinct modifications of hydrogen, they can interconvert under certain conditions, such as with the use of catalysts, electrical discharge, or high temperatures. Essentially pure para-hydrogen can be produced by cooling the mixture with charcoal, converting all ortho-hydrogen to para-hydrogen. The two forms have slightly different physical properties, including melting and vapor pressures.2