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A new channel in ortho-para hydrogen conversion is suggested as a tenative interpretation of large experimental reaction rates. This process occurs in two steps, via a virtually excited electronic state. The first step involves an exchange of electrons between the molecule and a nearby magnetic impurity while the second step is due to the hyperfine contact interaction between the virtually excited electronic spin of the molecule and its nuclear spins. This new process turns out to be more effective than the Wigner one if the exchange interaction exceeds a moderate value. PACS numbers: 82.65.Jv, 31.30.Gs, 75.30.Et Orthohydrogen and parahydrogen (o-JHb,/?-!^) considered as two varieties of molecular hydrogen, are associated with the early history of quantum mechanics. Since then, a tremendous number of o-p -H2 conversion experiments have been performed. The magnetic mechanism proposed by Wigner in 1933 1 is considered as a prototype of physical catalysis and is naturally distinguished from the simple chemical reaction where the two protons dissociate and recombine, as, for example, on metallic surfaces. Let us concentrate on the magnetic process. As solid catalysts, oxides provide a natural framework for the experimental test of the theory. Diluted impurities of chromia in an alumina support have been extensively considered, 2,3 as well as the whole series of sesquioxides of the rare earths supported (or not) on the diamagnetic lanthana. 4,5 For all oxides, the observed rates are found to be around 2 orders of magnitude larger than the theoretical rates, as deduced from the Wigner model. 2,4 Although it would be possible to account for this discrepancy by adjustment of the kinetics of molecular adsorption (an increase of the sticking time, for example), the parameter values that fit the experimental results seem unlikely. We therefore suggest an alternative channel for the magnetic 0-P-H2 conversion based on the virtual excitation of a magnetic molecular electronic state.As the H2 ground state has no electron spin, the magnetic Wigner mechanism for o-p conversion 1,6 " 10 is due to a hyperfine dipolar interaction between an external impurity spin and the two H2 nuclear spins. The inhomogeneous magnetic field, created by the impurity, pro-duces a relative dephasing in the precession of the two nuclear spins. This mechanism is effective as it satisfies the two needed selection rules AL odd,imposed by Pauli's principle, where L and / are the rotational and spin quantum numbers of the two nuclei. In this, hereafter labeled W, process, the selection rules (1) and (2) are simultaneously satisfied in one step, whereas we suggest that two-step processes, in which (1) and (2) are successively satisfied, may be at least as effective as the former. The first step, X, induces a virtual electronic transition due to an exchange coupling between the impurity electrons and those of the H2 molecule. This singlet-triplet transition introduces an electronic spin in the molecular intermediate state and allows the second step, Y, to...

Electron-energy-loss experiments of H2 physisorbed on noble metals at low temperature indicate that the ortho-para conversion rate is 2 orders of magnitude faster on a Ag(l 11) surface than on a Cu(lOO) one. We suggest a process in which a metal electron is virtually transferred back and forth from a surface band to the molecule antibonding orbital, the ortho-para energy being dissipated by metal (electron-hole) triplet pairs. The ortho-para rates are about 1 min on Ag(lll) and 1 h on Cu(lOO), in agreement with experimental data. This large difference does not characterize Ag and Cu, but is inherent to different surface orientations.