Abstract: SABRE (Signal amplification by reversible exchange) belongs to the family of methods for induced hydrogen-parahydrogen polarization of nuclei (PHIP). Pure parahydrogen potentially contains a huge stock of nonequilibrium polarization, and it can be stored for up to several weeks. One of the problems in PHIP applications is the singlet-triplet conversion in the H2 molecule, which reduces the lifetime of the singlet spin order of H2 in solution. In the presence of a catalyst, this conversion is significantly accelerated (up to several seconds), which negatively affects the enhancement of NMR signals. It became apparent that, despite the significant importance of the para-ortho conversion process in PHIP, there is a lack of systematic data in the literature regarding the relationship between the conversion rate and the magnetic field, as well as a consistent theoretical explanation of the conversion mechanism. In order to address this gap, we conducted both experimental and theoretical research on this correlation. Our findings allowed us to identify the factors influencing the decrease of singlet spin order lifetime of H2 nuclei and, ultimately, suggested methods to enhance the level of spin polarization and the intensity of NMR signals in PHIP and SABRE experiments utilizing parahydrogen. In this study, we examined the dynamics of para-ortho transitions of molecular hydrogen when dissolved in a solution with the presence of a SABRE catalyst. Para-ortho conversion is an important process and has previously been studied in the presence of paramagnetic metal ions to develop efficient catalysts for the liquefaction of molecular hydrogen (e.g., MOFs with paramagnetic metal ions [1]). Methods We introduced a new experimental method utilizing magnetic field cycling (Fig. 1) to assess the rate of para-ortho conversion of molecular hydrogen in a solution and utilized it in non-hydrogenative PHIP signal enhancement through reversible exchange (SABRE) experiments. The para-ortho conversion rate was determined across a broad magnetic field range spanning from 0.5 mT to 9.4 T. The research revealed that the conversion rate is significantly influenced by the magnetic field within which the reaction takes place, as well as by the concentrations of the reactants. The rate decreases as the concentration of the pyridine ligand increases and increases as the concentration of the iridium catalyst rises. It might be expected that the intensity of the NMR signal from orthohydrogen would rise and reach at a constant value, as there is initially little orthohydrogen in the system and its amount increases as the conversion progresses. However, this anticipated trend is not observed because the conversion of parahydrogen leads to the production of hyperpolarized orthohydrogen, whose signal intensity decreases exponentially toward the thermally equilibrated hydrogen signal intensity as the concentration of parahydrogen in the system decreases. Nevertheless, we believe that this observed exponential decay accurately reflects the rate of conversion from para- to orthohydrogen in the magnetic field with which the delay persists. Figure 1. Schematic of experiments to measure the field dependences. τbubble – time of sample bubbling with parahydrogen (10 s), τrel – time of varying delay The theoretical model considers the reversible exchange of molecular hydrogen with the catalyst, the nuclear spin-spin interaction of hydride protons with the nuclei of ligands within the catalytic complex, and the nuclear Zeeman interactions, providing a qualitative description of the experimental results. To calculate the para-ortho conversion rate, we have constructed and solved equations [2] on hydrogen density operators in free and bound forms, taking into account both spin dynamics and exchange between components. The conversion of molecular hydrogen is influenced by two types of complexes with different spin system symmetry. In asymmetric complexes with hydride protons having distinct chemical shifts due to the presence of a chlorine anion ligand, the para-ortho conversion rate increases with the magnetic field, while this mechanism does not apply to symmetric complexes. A resonant feature in the rate of para-ortho conversion is observed in the magnetic field where the level anti-crossing takes place. The findings of this study can be used to determine the optimal conditions that lead to maximum hyperpolarization in experiments using parahydrogen. Acknowledgements This work is supported by the Russian Science Foundation (Contract No. 23-73-10103).

Cite: Snadin A.V. , Kiryutin A.S. , Lukzen N.N. , Yurkovskaya A.V.
Singlet-triplet conversion in molecular hydrogen on a homogeneous catalyst in parahydrogen induced polarization experiments
21th International School-Conference MAGNETIC RESONANCE AND ITS APPLICATIONS Spinus-2024 01-05 Apr 2024