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

Funding information ChinaScholarshipCouncil,NationalNatural ScienceFoundationofChina,Grant/Award Number: 11604118;ChinaPostdoctoral ScienceFoundation,Grant/AwardNumber: 2015M581390 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 O2, but a detailed and quantitative understanding of the conversion process and its temperature dependencewas 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 O2 and the nuclear spins in H2 and the Fermi contact coupling from spin densities at the H-nuclei induced by O2. The latter were extracted from ab initio electronic structure calculations. State-to-state conversion cross sections and rate coefficients are obtained from quantummechanical coupled-channel calculations including the full anisotropic O2-H2 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 O2-H2 gas mixtures and explains the temperature dependence observed in the 1933measurements mentioned above.