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Author(s):van der Avoird, Ad
Abstract:It is well known among spectroscopists that two modifications of hydrogen exist: para-H$_2$ and ortho-H$_2$. Pure para-H$_2$ can be produced by leading `normal' H$_2$, a 1:3 para:ortho mixture, over an iron-containing catalyst at low temperature, and can be kept for a long time also at higher temperature in specially prepared gas cylinders. It is perhaps less well known that para-ortho H$_2$ conversion is also accelerated by interactions with paramagnetic molecules, such as O$_2$. 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 through a phenomenon called para-hydrogen induced polarization (PHIP). The para-ortho H$_2$ conversion by O$_2$ was recently measured in view of this application.[1] Two mechanisms have been suggested for the para-ortho H$_2$ conversion by collisions with O$_2$. The first one, proposed in 1933 by Eugene Wigner,[2] is the magnetic dipole-dipole coupling between the electron spin of O$_2$ and the nuclear spins of the two protons in H$_2$. 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, proposed by Minaev and {\AA}gren[3] in 1995, is that the overlap of the O$_2$ and H$_2$ wavefunctions in a collision complex transfers some of the spin density of O$_2$ to the wavefunction of H$_2$. The spin densities induced at the two H-nuclei may be different, which causes a different hyperfine interaction through the Fermi contact term. Wigner made a crude estimate of the para-ortho H$_2$ conversion rate with the use of some kinetic gas data. Minaev and {\AA}gren suggested, however, that the second mechanism is much more effective. We investigated the para-ortho H$_2$ conversion by collisions with O$_2$ by a first principles approach. Both mechanisms are included: the corresponding coupling terms are quantitatively evaluated as a function of the geometry of the O$_2$-H$_2$ collision complex by means of \textit{ab initio} electronic structure calculations. Then they are included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O$_2$ and H$_2$, which yield the para-ortho H$_2$ conversion cross sections and the rate coefficients for a range of temperatures. The conversion rate at room temperature is compared with the value measured in H$_2$-O$_2$ gas mixtures.[1] [1] S. Wagner, Magn. Reson. Mater. Phys., Biol. Med. {\bf 27}, 195 (2014). [2] E. Wigner, Z. Phys. Chem. B {\bf 23}, 28 (1933). [3] B. F. Minaev and H. {\AA}gren, J. Phys. Chem {\bf 99}, 8936 (1995).
Issue Date:25-Jun-20
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Date Available in IDEALS:2020-06-26

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