Polychromatic excitation for ¹H SABRE polarization transfer in weakly coupled systems at high field

Signal Amplification By Reversible Exchange (SABRE) increases NMR sensitivity using parahydrogen as a source of nuclear spin polarization. In SABRE, the to-be-polarized substrate and dihydrogen in bulk form a transient polarization transfer complex (PTC). This study reports high-field ¹H polarization transfer via SABRE induced by selective polychromatic excitation of ¹H nuclear spins, avoiding a magnetic field-cycling setup and high-power RF pulses. This enables routine SABRE implementation on standard NMR equipment, including MRI scanners. The proposed polychromatic excitation is efficient for the ¹H polarization transfer in the PTCs comprising two weakly coupled hydride ¹H nuclei, i.e. with a large chemical shift difference γ2πB₀Δδ≫J. We show that in a single type of the PTC, coherent high-field ¹H polarization transfer is driven by double-RF excitation, i.e. applied at two different frequencies simultaneously. In the general case of several PTCs formed by the substrate molecule, the double-RF excitation allows to selectively induce the ¹H polarization transfer in the PTC of interest. To maximize the polarization levels achieved when the substrate forms several types of the PTCs, we propose multi-RF excitation, i.e. applied at more than two ¹H frequencies simultaneously, as a generalization of the double-RF approach. The maximum ¹H signal enhancements at 9.4 T achieved are as follows: -27 for free nicotinamide (-85 for complex-bound), -21 for free 3-methylpyridine (-105 for complex-bound), -23 for pyridine. Remarkably, for all substrates the ¹H signal enhancement exceeds that obtained spontaneously in the high-field: ∼3-9 -fold for the free substrates and ∼10-20-fold for the complex-bound substrates.