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 (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 Î_1zÎ_2z spin order of catalyst-bound H₂; (ii) spin order conversion Î_1zÎ_2z→(Î_1z+Î_2z) 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.