Spin isomers of hydrogen

Molecular hydrogen occurs in two isomeric forms, one with its two proton spins aligned parallel (orthohydrogen), the other with its two proton spins aligned antiparallel (parahydrogen). These two forms are often referred to as spin isomers, since they differ not in chemical structure (like most isomers) but rather in nuclear spin state.

Parahydrogen is in a lower energy state than is orthohydrogen. At room temperature and thermal equilibrium, thermal excitation causes hydrogen to consist of approximately 75% orthohydrogen and 25% parahydrogen. When hydrogen is liquified at low temperature, there is a slow spontaneous transition to a predominantly para ratio, with the released energy having implications for storage. Essentially pure parahydrogen form can be obtained at very low temperatures, but it is not possible to obtain a sample containing more than 75% orthohydrogen by cooling.

Each hydrogen molecule (H₂) consists of two hydrogen atoms linked by a covalent bond. If we neglect the small proportion of deuterium and tritium which may be present, each hydrogen atom consists of one proton and one electron. Each proton has an associated magnetic moment, which is associated with the proton's spin of 1/2. In the H₂ molecule, the spins of the two hydrogen nuclei (protons) couple to form a triplet state known as orthohydrogen, and a singlet state known as parahydrogen.

The triplet orthohydrogen state has total nuclear spin I = 1 so that the component along a defined axis can have the three values M_I = 1, 0, or −1. The corresponding nuclear spin wavefunctions are ${\displaystyle |\uparrow \uparrow \rangle ,(1/{\sqrt {2}})(|\uparrow \downarrow \rangle +|\downarrow \uparrow \rangle )}$ and ${\displaystyle |\downarrow \downarrow \rangle }$ (in standard bra–ket notation). Each orthohydrogen energy level then has a (nuclear) spin degeneracy of three, meaning that it corresponds to three states of the same energy (in the absence of a magnetic field). The singlet parahydrogen state has nuclear spin quantum numbers I = 0 and M_I = 0, with wavefunction ${\displaystyle (1/{\sqrt {2}})(|\uparrow \downarrow \rangle -|\downarrow \uparrow \rangle )}$. Since there is only one possibility, each parahydrogen level has a spin degeneracy of one and is said to be non degenerate.

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