Para‐ortho hydrogen conversion: Solving a 90‐year old mystery
Xia Zhang, Tijs Karman, Gerrit C. Groenenboom, Ad van der Avoird
Abstract
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, 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 and the nuclear spins in H and the Fermi contact coupling from spin densities at the H‐nuclei induced by O. 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‐H 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‐H 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