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Issue 16, 2006
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Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

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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.

Graphical abstract: Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

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Publication details

The article was received on 02 Feb 2006, accepted on 28 Feb 2006 and first published on 14 Mar 2006

Article type: Paper
DOI: 10.1039/B601594H
Phys. Chem. Chem. Phys., 2006,8, 1929-1935

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    Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

    G. Buntkowsky, B. Walaszek, A. Adamczyk, Y. Xu, H.-H. Limbach and B. Chaudret, Phys. Chem. Chem. Phys., 2006, 8, 1929
    DOI: 10.1039/B601594H

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