title: Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion.
abstract: In this paper a quantitative explanation for a diamagnetic ortho/para H2 conversion is given. The description is based on the quantum-mechanical density matrix formalism originally developed by Alexander and Binsch for studies of exchange processes in NMR spectra. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a three spin system, the reactions of the hydrogen gas with the catalysts are treated as a phenomenological rate process, described by a rate constant. Numerical calculations reveal that for nearly all possible geometrical arrangements of the three spin system an efficient spin conversion is obtained. Only in the chemically improbable case of a linear group H-X-H no spin conversion is obtained. The efficiency of the spin conversion depends strongly on the lifetime of the H-X-H complex and on the presence of exchange interactions between the two hydrogens. Even moderate exchange couplings cause a quench of the spin conversion. Thus a sufficiently strong binding of the dihydrogen to the S spin is necessary to render the quenching by the exchange interaction ineffective.
authors.affiliation: FSU Jena, Institut für Physikalische Chemie, Helmholtzweg 4, 07743, Jena, Germany. gerd.buntkowsky@uni-jena.de
authors.forename: G
authors.initials: G
authors.lastname: Buntkowsky
authors.forename: B
authors.initials: B
authors.lastname: Walaszek
authors.forename: A
authors.initials: A
authors.lastname: Adamczyk
authors.forename: Y
authors.initials: Y
authors.lastname: Xu
authors.forename: H-H
authors.initials: HH
authors.lastname: Limbach
authors.forename: B
authors.initials: B
authors.lastname: Chaudret
mesh_terms: D002384:Catalysis; D004578:Electron Spin Resonance Spectroscopy; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008433:Mathematics; D008956:Models, Chemical
chemical_list: D006859:Hydrogen