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https://doi.org/10.5194/mr-2020-16
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/mr-2020-16
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 26 Jun 2020

Submitted as: research article | 26 Jun 2020

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This preprint is currently under review for the journal MR.

Geminal Parahydrogen-Induced Polarization: Accumulating Long-Lived Singlet Order on Methylene Proton Pairs

Laurynas Dagys1,, Barbara Ripka1,, Markus Leutzsch2, Gamal A. I. Moustafa1, James Eills3, Johannes F. P. Colell1, and Malcolm H. Levitt1 Laurynas Dagys et al.
  • 1School of Chemistry, University of Southampton, SO17 1BJ, UK
  • 2Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany
  • 3Helmholtz Institute Mainz, Johannes Gutenberg University, D-55099 Mainz, Germany
  • These authors contributed equally to this work.

Abstract. In the majority of hydrogenative PHIP (Parahydrogen Induced Polarization) experiments, the hydrogen molecule undergoes pairwise cis-addition to an unsaturated precursor to occupy vicinal positions on the product molecule. However, some ruthenium-based hydrogenation catalysts induce geminal hydrogenation, leading to a reaction product in which the twohydrogen atoms are transferred to the same carbon center, forming a methylene (CH2) group. The singlet order of parahydrogen is substantially retained over the geminal hydrogenation reaction, giving rise to a singlet-hyperpolarized CH2 group. Although the T1 relaxation times of the methylene protons are often short, the singlet order has a long lifetime, providing that singlet-triplet mixing is suppressed, either by chemical equivalence of the protons or by applying a resonant radiofrequency field. The long lifetime of the singlet order enables the accumulation of hyperpolarization during the slow hydrogenation reaction. We introduce a kinetic model for the behaviour of the observed hyperpolarized signals, including both the chemical kinetics and the spin dynamics of the reacting molecules. Our work demonstrates the feasibility of producing singlet-hyperpolarized methylene moieties by parahydrogen-induced polarization. This potentially extends the range of molecular agents which maybe generated in a hyperpolarized state by chemical reactions of parahydrogen.

Laurynas Dagys et al.

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Short summary
We explored hydrogenation reaction with para-spin isomer of hydrogen molecule as a source of nuclear singlet state. As a product of this reaction, the state was populated between two protons of the methylene group. We have shown that utilizing the long-lived properties of such nuclear spin state a high nuclear spin polarization can be accumulated. To explain this effect we have given a kinetic model which could be potentially applied for similar schemes involving spin dynamics in chemistry.
We explored hydrogenation reaction with para-spin isomer of hydrogen molecule as a source of...
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