Mechanism of spontaneous polarization transfer in high-field SABRE experiments

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Abstract

We propose an explanation of the previously reported SABRE (Signal Amplification By Reversible Exchange) effect at high magnetic fields, observed in the absence of RF-excitation and relying only on “spontaneous” polarization transfer from parahydrogen (pH2, the H2 molecule in its nuclear singlet spin state) to a SABRE substrate. We propose a detailed mechanism for spontaneous polarization transfer and show that it is comprised of three steps: (i) Generation of the anti-phase Î1zÎ2z spin order of catalyst-bound H2; (ii) spin order conversion Î1zÎ2z→(Î1z2z) due to cross-correlated relaxation, leading to net polarization of H2; (iii) polarization transfer to the SABRE substrate, occurring due to NOE. Formation of anti-phase polarization is due to singlet-to-T0 mixing in the catalyst-bound form of H2, while cross-correlated relaxation originates from fluctuations of dipole–dipole interactions and chemical shift anisotropy. The proposed mechanism is supported by a theoretical treatment, magnetic field-dependent studies and high-field NMR measurements with both pH2 and thermally polarized H2.

Original languageEnglish
Pages (from-to)74-81
Number of pages8
JournalJournal of Magnetic Resonance
Volume287
DOIs
Publication statusPublished - 1 Feb 2018

Keywords

  • PARAHYDROGEN INDUCED POLARIZATION
  • NMR SIGNAL AMPLIFICATION
  • PARA-HYDROGEN
  • REVERSIBLE EXCHANGE
  • MAGNETIC-FIELD
  • N-15 HYPERPOLARIZATION
  • CROSS-CORRELATIONS
  • SPIN POLARIZATION
  • RELAXATION
  • KINETICS

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