Mechanism of spontaneous polarization transfer in high-field SABRE experiments

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 (pH(2), the H-2 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 (I) over cap (1z)(I) over cap (2z) spin order of catalyst bound H-2; (ii) spin order conversion (I) over cap (1z)(I) over cap (2z) -> ((I) over cap (1z) + (I) over cap (2z)) due to cross-correlated relaxation, leading to net polarization of H-2; (iii) polarization transfer to the SABRE substrate, occurring due to NOE. Formation of anti-phase polarization is due to singlet-to-T-0 mixing in the catalyst-bound form of H-2, 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 pH(2) and thermally polarized H-2. (C) 2017 Elsevier Inc. All rights reserved.