Why NO is much more efficient than O₂ in para-ortho H₂ conversion

Molecular hydrogen (H₂) exists in two states: para-H₂ and ortho-H₂, determined by the spin of the nuclei. Para-H₂ is widely used as rocket fuel and to increase the sensitivity of nuclear magnetic resonance, so it is important to understand the mechanics of para-ortho conversion.

Ad van der Avoird and Gerrit C. Groenenboom revealed the mechanism of para-ortho H₂ conversion by nitric oxide (NO) and explained why NO was more effective at this process than molecular oxygen (O₂).

Experimental physicists published this result in the 1930s, but the actual reason behind the different conversion rates with NO and O₂ remained murky until now. Using quantum calculations that considered the coupling between the magnetic dipole moments of NO and of the hydrogen nuclei, as well as the Fermi contact coupling determined by the spin densities in H₂, the researchers found the key to this difference: strong resonances.

Near-resonant collisions of para-H₂ with NO in its upper spin-orbit state de-excite NO to the ground state, which releases the perfect amount of energy to convert para-H₂ into ortho-H₂. The conversion is mediated by the transient magnetic dipole moment of NO occurring during these NO-H₂ collisions.

The researchers calculated conversion rate coefficients for temperatures up to 200K, which quantitatively agreed with the data collected in the 1930s.

Avoird said that this paper presents a very satisfactory conclusion to his work on this topic.

“My first goal was to understand how the conversion occurs,” he said. “But since I also explained the difference between O₂ and NO now, I’m very happy, and I think I can leave this topic.”

Source: “Para-ortho H₂ conversion in collisions with NO: A surprising mechanism,” by Ad van der Avoird and Gerrit C. Groenenboom, Journal of Chemical Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0345867 .