Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion
Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion
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DOI：
10.1039/b601594h
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发表时间：
2006-01-01
影响因子：
3.3
通讯作者：
Chaudret, B
中科院分区：
文献类型：
--
作者：
Buntkowsky, G;Walaszek, B;Chaudret, B
本文对反磁性H-2的邻位/对位转换作了定量解释。描述是基于量子力学密度矩阵形式主义最初开发的亚历山大和宾士的研究交换过程的NMR光谱。只有核自旋系统被量子力学处理。采用三自旋系统模型，氢气与催化剂的反应被视为一个唯象速率过程，用速率常数来描述。数值计算表明，对于几乎所有可能的三自旋系统的几何排列，都能得到有效的自旋转换。只有在化学上不可能的线性基团H-X-H的情况下，才不会得到自旋转换。自旋转换的效率在很大程度上取决于H-X-H络合物的寿命和两个氢之间交换相互作用的存在。即使是中等的交换偶联也会引起自旋转化的淬灭。因此，二氢与S自旋的足够强的结合是使交换相互作用的猝灭无效所必需的。
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.