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Title:Para-ortho Hydrogen Conversion; Solving A 90-year Old Mystery
Author(s):van der Avoird, Ad
Abstract:It is well known among spectroscopists that hydrogen has two modifications: para-H$_2$ and ortho-H$_2$. Pure para-H$_2$ can be produced by leading ``normal'' H$_2$, a 3:1 ortho:para mixture, over a catalyst at low temperature. It is perhaps less well known that para-ortho H$_2$ conversion is also catalyzed by collisions with paramagnetic molecules, such as O$_2$. Almost ninety years ago Farkas and Sachsse measured the rate coefficient of para-ortho H$_2$ conversion in gas mixtures with O$_2$.[1] In the same year, 1933, it was proposed by Wigner [2] that it is the magnetic dipole-dipole coupling between the electron spin of O$_2$ and the nuclear spins of the two protons in H$_2$ 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-H$_2$, as well as their rotational states with even and odd $j$ values. Another mechanism, suggested to be much more effective, was proposed later: the exchange interaction with the open-shell O$_2$ induces spin density into the electronic wavefunction of H$_2$. In most collisions the spin density is different at the two H-nuclei, which makes them inequivalent by different hyperfine interactions through the Fermi contact term. An important application of para-H$_2$ is in NMR spectroscopy and its imaging variant, MRI. By adding para-H$_2$ to the sample the sensitivity of NMR can be increased by four orders of magnitude by a phenomenon called para-hydrogen induced polarization (PHIP). Para-ortho H$_2$ conversion by O$_2$ in the gas phase was remeasured in 2014 in view of this application. A detailed and quantitative understanding of the conversion process was still lacking, however. We theoretically investigated the para-ortho H$_2$ conversion by collisions with O$_2$ in a first principles approach.[3] Both mechanisms were taken into account and the corresponding coupling terms were quantitatively evaluated as functions of the geometry of the O$_2$-H$_2$ collision complex by means of \textit{ab initio} electronic structure calculations. Then they were included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O$_2$ and H$_2$, which yielded the para-ortho H$_2$ 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 H$_2$-O$_2$ gas mixtures. The calculations provide detailed insight into the conversion process. [1] L. Farkas and H. Sachsse, Z. Phys. Chem. B {\bf 23}, 1 (1933). [2] E. Wigner, Z. Phys. Chem. B {\bf 23}, 28 (1933). [3] X. Zhang, T. Karman, G.~C. Groenenboom, and A. van der Avoird, Nat. Sci. (2021);
Issue Date:2021-06-21
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper / Presentation
Date Available in IDEALS:2021-09-24

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