Ortho and Para-Hydrogen

Every hydrogen nucleus (a lone proton) has a nuclear spin. When two hydrogen atoms combine to form the H₂ molecule, the spins of the two nuclei can point in the same direction or in opposite directions -- and because the two arrangements are physically distinguishable, they are treated as two different forms, or nuclear-spin isomers, of the H₂ molecule:

• Ortho-hydrogen -- the two nuclear spins are parallel (pointing the same way).
• Para-hydrogen -- the two nuclear spins are antiparallel (pointing opposite ways).

At room temperature, an ordinary sample of hydrogen gas is a mixture of about 75% ortho-form and 25% para-form. Because the ortho form is the more stable of the two at ordinary temperatures, the two isomers only slowly interconvert on their own. Lowering the temperature, however, shifts the equilibrium in favour of the para form.

The para form can be converted back into the ortho form in several ways: catalytically, using finely divided platinum or iron; by passing an electric discharge through the gas; by heating above 800°C; or by mixing it with paramagnetic species -- molecules such as O₂, NO or NO₂, or with nascent (freshly generated, atomic) hydrogen -- which catalyse the spin flip. …