orthopara

Orthopara refers to a set of nuclear spin isomers arising in molecules that contain pairs of identical spin-1/2 nuclei, most commonly protons. In these systems, two distinct quantum states exist: ortho, where the spins are parallel, and para, where the spins are antiparallel. The designation reflects how spin symmetry combines with the molecule’s rotational states under the Pauli exclusion principle.

In the classic case of molecular hydrogen, H2, the relation between spin state and rotational state is well defined. The ortho form (triplet spin state) couples with odd rotational quantum numbers, while the para form (singlet spin state) couples with even rotational quantum numbers. At a given temperature, the populations of these forms follow Boltzmann statistics, and the equilibrium ortho-to-para ratio tends toward a temperature-dependent value; at high temperatures it approaches about 3:1, reflecting the degeneracy of the three triplet spin states.

The concept extends to other molecules with multiple protons, such as water (H2O), ammonia (NH3), and methane (CH4). In these species, ortho and para forms also arise from nuclear spin symmetry, but the specific energy-level spacings and selection rules differ, leading to different equilibrium ratios and interconversion behaviors. Observations of ortho-para ratios can provide information about formation conditions, thermal history, and processes in environments ranging from laboratory reactors to interstellar space.

Interconversion between ortho and para forms can be slow in the gas phase, often requiring catalysts, surfaces, or magnetic interactions to proceed readily. Because the ratios depend on temperature and history, orthopara isomerism remains a useful probe in spectroscopy, thermodynamics, and astrochemistry.