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Mechanism of nuclear spin initiated para-H-2 to ortho-H-2 conversion

Buntkowsky, G. and Walaszek, B. and Adamczyk, A. and Xu, Y. and Limbach, H. H. and Chaudret, B. (2006):
Mechanism of nuclear spin initiated para-H-2 to ortho-H-2 conversion.
In: Physical Chemistry Chemical Physics, 8 (16), pp. 1929-1935, [Online-Edition: http://apps.webofknowledge.com/full_record.do?product=WOS&se...],
[Article]

Abstract

In this paper a quantitative explanation for a diamagnetic ortho/para H-2 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.

Item Type: Article
Erschienen: 2006
Creators: Buntkowsky, G. and Walaszek, B. and Adamczyk, A. and Xu, Y. and Limbach, H. H. and Chaudret, B.
Title: Mechanism of nuclear spin initiated para-H-2 to ortho-H-2 conversion
Language: English
Abstract:

In this paper a quantitative explanation for a diamagnetic ortho/para H-2 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.

Journal or Publication Title: Physical Chemistry Chemical Physics
Volume: 8
Number: 16
Uncontrolled Keywords: parahydrogen-induced polarization molecular-hydrogen complexes state nmr-spectroscopy situ phip-nmr homogeneous hydrogenation magnetic-resonance oxidative addition stereoselective hydrogenation pairwise addition line-shapes
Divisions: 07 Department of Chemistry
07 Department of Chemistry > Physical Chemistry
Date Deposited: 27 Oct 2014 20:38
Official URL: http://apps.webofknowledge.com/full_record.do?product=WOS&se...
Additional Information:

032KB Times Cited:32 Cited References Count:56

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