spinisomeren

Spin isomerism, or spin isomerism of molecules, is the phenomenon where molecules with the same chemical formula occur in distinct nuclear spin configurations that are not interconvertible without breaking bonds or supplying energy. It is a type of isomerism based on the spin state of identical nuclei, typically protons, in a molecule.

In molecules containing two or more identical nuclei with nonzero spin, the combined nuclear spin can take different values. Because identical fermions (such as protons with spin 1/2) must obey quantum statistics, the overall molecular wavefunction is symmetric or antisymmetric under exchange, linking spin state to rotational state. This yields distinct isomeric species, commonly labeled ortho and para; in systems with more complex spin networks, additional families (meta, etc.) can occur.

Common examples include molecular hydrogen (H2), which has ortho-H2 (parallel proton spins) and para-H2 (antiparallel); the high-temperature equilibrium ratio is approximately 3:1. Water (H2O) also exhibits ortho and para spin isomers due to the two equivalent protons. Interconversion between spin isomers can be slow in the gas phase at low temperatures and may be catalyzed by catalysts or surfaces.

Spin isomerism affects physical properties and spectra, including rotational spectra and thermodynamic populations, and plays a role in astrochemistry and cryogenics, where the spin-state distribution can influence heat capacity and reaction kinetics.