Physics:Quantum atoms/spin isomers of hydrogen

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Spin isomers of hydrogen are forms of molecular hydrogen that differ in the relative nuclear spin orientation of the two protons in ${\mathrm{H}}_2$. The two main forms are orthohydrogen and parahydrogen.

Ortho and para hydrogen

In orthohydrogen, the two proton spins are parallel and the total nuclear spin is $I=1$. In parahydrogen, the two proton spins are antiparallel and the total nuclear spin is $I=0$.

Because protons are fermions, the total molecular wavefunction must have the proper exchange symmetry. This links the nuclear spin state to the allowed rotational states of the molecule.

Quantum significance

The ortho and para forms have different rotational energy level populations and different low-temperature behavior. At room temperature hydrogen is mostly orthohydrogen, while at very low temperature the para form is energetically favored.

Spin isomers are important in cryogenics, molecular spectroscopy, nuclear spin physics, and precision studies of hydrogen molecules.

Related pages

• Physics:Quantum atoms/hydrogen
• Physics:Quantum atoms/isotopes of hydrogen
• Physics:Quantum Molecular orbital theory
• nuclear spin states
• Physics:Quantum fermion

References

• "Ortho-hydrogen and para-hydrogen". https://www.britannica.com/science/hydrogen/Ortho-hydrogen-and-para-hydrogen. 
• Silvera, Isaac F. (1980). The solid molecular hydrogens in the condensed phase: Fundamentals and static properties. 52. 393-452. doi:10.1103/RevModPhys.52.393. 

Author: Harold Foppele