Magnetic field dependence of the para-ortho conversion rate of molecular hydrogen in SABRE experiments

IF 2 3区 化学 Q3 BIOCHEMICAL RESEARCH METHODS Journal of magnetic resonance Pub Date : 2024-02-15 DOI:10.1016/j.jmr.2024.107630
Alexander V. Snadin , Natalia O. Chuklina , Alexey S. Kiryutin , Nikita N. Lukzen , Alexandra V. Yurkovskaya
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Abstract

The use of parahydrogen – the isomer of molecular hydrogen with zero nuclear spin – is important for promising and actively developing methods for spin hyperpolarization of nuclei called parahydrogen induced polarization (PHIP). However, the dissolved parahydrogen in PHIP experiments quickly loses its spin order, resulting in the formation of orthohydrogen and reduction of the overall nuclear polarization of the substrate. This process is due to the difference of chemical shifts of hydride protons, as well as spin–spin couplings between nuclei, in the intermediate catalytic complexes, and it has not been rigorously explained so far. We proposed a new experimental technique based on magnetic field cycling for measuring the rate of molecular hydrogen para–ortho conversion in solution and applied it for non-hydrogenative PHIP Signal Amplification By Reversible Exchange (SABRE) experiments. The para–ortho conversion rate was measured over a wide range of magnetic field from 0.5 mT to 9.4 T. It was found that the conversion rate strongly depends on the magnetic field in which the reaction occurs, as well as on the concentrations of reactants. The rate decreases with increasing the concentration of pyridine ligand and increases with increasing the concentration of iridium catalyst. The model, which takes into account the reversible exchange of molecular hydrogen with the catalyst, nuclear spin–spin interaction of hydride protons with nuclei of ligands within catalytic complex and nuclear Zeeman interactions, qualitatively describes the experimental data. Two types of complexes with different spin system symmetry contribute to the molecular hydrogen conversion. In asymmetric complexes possessing hydride protons with different chemical shifts due to the presence of chlorine anion ligand the para–ortho conversion rate increases with magnetic field, while for symmetric complexes this mechanism is not operable. In the magnetic field where level anti-crossing occurs the resonant feature for the rate of para–ortho conversion is found. The results of this work can be utilized for finding the optimal conditions for obtaining the maximum hyperpolarization in the experiments employing parahydrogen.

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SABRE 实验中分子氢的对正转换率与磁场的关系
对氢--核自旋为零的分子氢异构体--的使用对于有前景并正在积极开发的核自旋超极化方法(即对氢诱导极化(PHIP))非常重要。然而,PHIP 实验中溶解的对氢很快就会失去其自旋顺序,从而形成正氢并降低基底的整体核极化。这一过程是由于中间催化复合物中氢化物质子的化学位移以及原子核之间的自旋-自旋耦合的差异造成的,迄今为止还没有得到严格的解释。我们提出了一种基于磁场循环的测量溶液中分子氢对正转化率的新实验技术,并将其应用于非氢化 PHIP 可逆交换信号放大(SABRE)实验。在 0.5 mT 至 9.4 T 的宽磁场范围内测量了对位正交转化率。结果发现,转化率与发生反应的磁场以及反应物的浓度密切相关。转化率随吡啶配体浓度的增加而降低,随铱催化剂浓度的增加而升高。该模型考虑了分子氢与催化剂的可逆交换、氢化物质子与催化络合物内配体原子核的核自旋-自旋相互作用以及核泽曼效应,定性地描述了实验数据。两类具有不同自旋系统对称性的复合物有助于分子氢的转化。在非对称络合物中,由于氯阴离子配体的存在,氢化物质子具有不同的化学位移,对正转换率随磁场而增加,而对称络合物则不存在这种机制。在发生电平反交叉的磁场中,发现了对位正交转换率的共振特征。这项工作的结果可用于在使用对氢的实验中寻找获得最大超极化的最佳条件。
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来源期刊
CiteScore
3.80
自引率
13.60%
发文量
150
审稿时长
69 days
期刊介绍: The Journal of Magnetic Resonance presents original technical and scientific papers in all aspects of magnetic resonance, including nuclear magnetic resonance spectroscopy (NMR) of solids and liquids, electron spin/paramagnetic resonance (EPR), in vivo magnetic resonance imaging (MRI) and spectroscopy (MRS), nuclear quadrupole resonance (NQR) and magnetic resonance phenomena at nearly zero fields or in combination with optics. The Journal''s main aims include deepening the physical principles underlying all these spectroscopies, publishing significant theoretical and experimental results leading to spectral and spatial progress in these areas, and opening new MR-based applications in chemistry, biology and medicine. The Journal also seeks descriptions of novel apparatuses, new experimental protocols, and new procedures of data analysis and interpretation - including computational and quantum-mechanical methods - capable of advancing MR spectroscopy and imaging.
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