orthohydrogen

Ortho-hydrogen is one of the two nuclear spin isomers of molecular hydrogen (H2). It consists of two protons whose nuclear spins are aligned parallel, giving a total nuclear spin I = 1 (a triplet state). In contrast, para-hydrogen has antiparallel spins (I = 0).

Because H2 is a homonuclear diatomic molecule of fermions, the total molecular wavefunction must be antisymmetric under exchange of the two nuclei. The spin and rotational parts compensate: ortho-hydrogen corresponds to symmetric spin (I = 1) and occupies odd rotational quantum numbers J = 1, 3, ...; para-hydrogen corresponds to antisymmetric spin (I = 0) and occupies even J = 0, 2, ...

At high temperature, the statistical weights yield an ortho-to-para ratio of about 3:1. As the temperature is lowered, the equilibrium shifts toward para-hydrogen, and near the temperature of liquid hydrogen the mixture is predominantly para.

Interconversion between the forms in the gas phase is slow without catalysts, but it can be accelerated by paramagnetic surfaces or impurities that facilitate spin flips. The ortho-to-para conversion is exothermic, releasing heat as ortho converts to para; this has practical implications for the handling and storage of liquid hydrogen.

Knowledge of the ortho–para ratio is important in cryogenics, hydrogen storage, and certain spectroscopic or nuclear-spin techniques. Parahydrogen can be exploited for hyperpolarization in magnetic resonance and related applications, where enriched para-hydrogen enhances signal.