Para-ortho hydrogen conversion: Solving a 90-year old mystery

Abstract

Almost ninety years have passed since the experiments of Farkas and Sachsse [Z. Phys. Chem. B 1933; 23:1] on para-ortho hydrogen conversion catalyzed by paramagnetic species such as O (Formula presented.), but a detailed and quantitative understanding of the conversion process and its temperature dependence was still lacking. Here, we present a complete and quantitative theoretical treatment of this catalytic process. Both interactions causing the conversion are included: the magnetic dipole-dipole coupling between the electron spin of O (Formula presented.) and the nuclear spins in H (Formula presented.) and the Fermi contact coupling from spin densities at the H-nuclei induced by O (Formula presented.). The latter were extracted from ab initio electronic structure calculations. State-to-state conversion cross sections and rate coefficients are obtained from quantum mechanical coupled-channel calculations including the full anisotropic O (Formula presented.) -H (Formula presented.) interaction potential and by treating both the spin-dependent couplings perturbatively. The total rate coefficient agrees with the experimental value recently measured by Wagner [Magn. Reson. Mater. Phys., Biol. Med. 2014; 27:195] in O (Formula presented.) -H (Formula presented.) gas mixtures and explains the temperature dependence observed in the 1933 measurements mentioned above. Key points: two species of hydrogen, para and ortho conversion finally explained by ab initio theory para-hydrogen induced polarization in NMR and MRI.