New Channel in Ortho-Para Hydrogen Conversion

Abstract

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...

Magneto-Optic and Magneto-Catalytic Effect

“…The original Wigner mechanism arises in the coupling of the surface magnetic field gradient with the H2 nuclear spins difference [44]. However this coupling is small and an alternative channel has been,recently,suggested [45]. The underlying idea of this mechanism is the use of the magnetic field gradient created by the molecular electrons spins, as a substitute for the external one.…”

“…The Coulomb interaction between the molecule and impurity electrons induce the transition ( I~ + 4A2)~(3I+ + 4 A2 ) among the pair states. As the 3I~ is characterized by an antisymmetric orbital eigenstat~,CoulOmb interaction, restricted to the preceding electronic transition, is equivalent to an exchange coupling between the chromia electron spin and the molecule two-electrons spin difference [45]. It is worthwhile to stress that this non-dia~0nal exchange interaction induces, together, a rotational quantum jump of odd parity.…”

Electron Spin Polarization Transfer to ortho-H2 by Interaction of para-H2 with Paramagnetic Species: A Key to a Novel para → ortho Conversion Mechanism

“…Some discrepancies were noticed with respect to the Wigner theory, in particular concerning the distance between the hydrogen atoms and the unpaired 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 electron(s), 10,11 and several theoretical works have been published, proposing new mechanisms for the para→ortho conversion. [12][13][14] In the course of a 1 H NMR study dealing with 100% enriched p-H 2 , dissolved in organic solvents at room temperature and in the earth magnetic field (see Supporting Information (SI) for details), we came across a rather puzzling effect that has never been described in the literature (to the best of our knowledge). When a paramagnetic species (such as Cr(acac) 3 ) is added to a deuterated organic solvent (such as acetone-d 6 ), we observe that, upon insertion of the sample in the magnetic field of the NMR spectrometer, the o-H 2 NMR signal appears immediately but as a negative peak (Figure 1).…”

Direct 1H NMR evidence of spin-rotation coupling as a source of para → ortho-H2 conversion in diamagnetic solvents

“…13 When p-H 2 is dissolved in an organic solvent inside an NMR tube, the observed conversion kinetics can be relatively fast, with characteristic times that range rather from minutes to several hours, 5,10,11 and that typically decrease linearly with the concentration of spin catalysts within the solvent. 3,5,11,14 The mechanisms of para → ortho conversion have been investigated over many decades since the discovery of para-hydrogen, 10,[14][15][16][17][18][19] but they still remain a subject of discussion. 5,[10][11][12]20 As early as 1933, Wigner showed that the presence of magnetic species catalyses the conversion process.…”