Spin isomers of hydrogen

Each hydrogen molecule (H2) consists of two hydrogen atoms linked by a covalent bond. If we neglect the traces of deuterium and tritium which could be present, each hydrogen atom consists of one proton and one electron. The proton has an associated magnetic moment, which we can treat as being generated by the proton's spin. The spins of the two hydrogen atoms can either be aligned the same direction (this is orthohydrogen) or in opposite directions (this is parahydrogen). The ratio between the ortho and para forms is about 3:1 at standard temperature and pressure, but the para form dominates at low temperatures (approx. 99.95% at 20 K). Other molecules and functional groups containing two hydrogen atoms, such as water and methylene, also have ortho and para forms, although their ratios differ from that of the dihydrogen molecule.

Orthohydrogen is unstable at low temperatures and spontaneously changes into parahydrogen, but the process is slow because the kinetic barrier to interconversion is high. The conversion from ortho to para state is exothermic (releasing heat). The presence of a paramagnetic substance in liquid hydrogen can induce rapid heating - an undesirable occurrence when one wants hydrogen to remain liquid. At room temperature, hydrogen contains 75% orthohydrogen, a proportion which the liquefaction process preserves. One must therefore use a catalyst to accelerate the conversion of the liquid hydrogen into parahydrogen, or supply additional refrigeration equipment to absorb the heat that the liquid hydrogen will give off as it spontaneously converts itself to pure parahydrogen.

The first synthesis of pure parahydrogen was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.