Para-ortho H₂ conversion by collisions with O₂ and NO

It has been known since 1927 that two modifications of hydrogen exist: para-H₂ and ortho-H₂. Pure para-H₂ can be prepared by leading a 1:3 para:ortho mixture of “normal” H₂ over a catalyst at low temperature. In 1933 Farkas and Sachsse [1] measured the rate coefficient of para-ortho H₂ conversion in gas mixtures with the paramagnetic open-shell molecules O₂, NO, and NO₂. In the same year it was proposed by Wigner [2] that it is the magnetic dipole-dipole coupling between the electron spin of the paramagnetic molecule and the nuclear spins of the two protons in H₂ that is responsible for the conversion. In asymmetric collisions this coupling makes the two H-nuclei inequivalent and mixes the nuclear spin functions of para- and ortho-H₂, as well as their rotational states with even and odd j values. Another mechanism was proposed later and suggested to be much more effective: the exchange interaction with the open-shell molecule induces spin density into the electronic wave function of H₂, which leads to different hyperfine interactions with the H-nuclei through the Fermi contact term.
A currently widespread application of para-H₂ is in NMR spectroscopy. By adding para-H₂ to the sample the sensitivity of NMR can be increased by four orders of magnitude by a phenomenon called para-hydrogen induced polarization (PHIP).
We theoretically investigated [3] the para-ortho H₂ conversion by collisions with O₂ in a first principles approach. Both mechanisms were taken into account and the corresponding coupling terms quantitatively evaluated as functions of the geometry of the O₂-H₂ collision complex by means of ab initio electronic structure calculations. Then they were included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O₂ and H₂, which yielded the para-ortho H₂ conversion cross sections and the rate coefficients for temperatures up to 400 K. The conversion rate and its temperature dependence are in good agreement with the values measured in O₂-H₂ gas mixtures. The calculations provide detailed insight into the conversion process.
Surprisingly Farkas and Sachsse [1] found in 1933 that NO is 3 to 12 times more effective in para-ortho H₂ conversion than O₂. We are currently investigating NO-H₂ collisions theoretically. The theory and the calculations are much more difficult than for O₂-H₂, because NO has two spin-orbit states: ²Π_1/2 and ²Π_3/2, it has orbital angular momentum, and the spin is strongly coupled to the NO axis by spin-orbit coupling. Moreover, the ²Π state is spatially degenerate, so that non-adiabatic coupling effects are important. Our first results are interesting and but do not yet explain the peculiar activity of NO versus O₂.
References
[1] L. Farkas and H. Sachsse, 1, 23, Z. Phys. Chem. B (1933).
[2] E. Wigner, 28, 23, Z. Phys. Chem. B (1933).
[3] X. Zhang, T. Karman, G. C. Groenenboom, A. van der Avoird, e10002, 1, Nat. Sci. (2021)