parahydrogen

Parahydrogen is the nuclear spin isomer of molecular hydrogen (H2) in which the two protons have antiparallel spins, forming a singlet state with total nuclear spin I = 0. The complementary isomer, orthohydrogen, has parallel spins (I = 1) and a triplet spin state. Because hydrogen nuclei are fermions, the overall molecular wavefunction must be antisymmetric under exchange, linking spin and rotational motion.

This coupling means para hydrogen is associated with even rotational quantum numbers (J = 0, 2, 4, ...), while ortho hydrogen corresponds to odd rotational levels (J = 1, 3, 5, ...). At a given temperature, the population distribution between these spin isomers arises from the combined spin and rotational degeneracies.

In thermodynamic equilibrium, the ortho:para ratio depends on temperature. At room temperature the mixture is roughly 3:1 in favor of orthohydrogen, reflecting the three degenerate triplet spin states versus one singlet. As temperature decreases, the equilibrium shifts toward parahydrogen, and at very low temperatures the para fraction becomes dominant, approaching near-pure para in the ground rotational state (J = 0).

Interconversion between para and ortho forms is slow in the gas phase under ordinary conditions but can be catalyzed by surfaces or by paramagnetic species. Catalysts, impurities, or cryogenic conditions can accelerate the conversion.

Parahydrogen is of interest in cryogenics and in magnetic resonance techniques because its spin order can be harnessed to enhance signals. A notable application is parahydrogen-induced polarization (PHIP), which transfers parahydrogen spin order to other molecules to boost NMR or MRI sensitivity.