Coherent Evolution of Signal Amplification by Reversible Exchange in Two Alternating Fields (alt-SABRE)

Parahydrogen (pH(2)) is a convenient and cost-efficient source of spin order to enhance the magnetic resonance signal. Previous work showed that transient interaction of pH(2) with a metal organic complex in a signal amplification by reversible exchange (SABRE) experiment enabled more than 10 % polarization for some N-15 molecules. Here, we analyzed a variant of SABRE, consisting of a magnetic field alternating between a low field of similar to 1 mu T, where polarization transfer is expected to take place, and a higher field >50 mu T (alt-SABRE). These magnetic fields affected the amplitude and frequency of polarization transfer. Deviation of a lower magnetic field from a "perfect" condition of level anti-crossing increases the frequency of polarization transfer that can be exploited for polarization of short-lived transient SABRE complexes. Moreover, the coherences responsible for polarization transfer at a lower field persisted during magnetic field variation and continued their spin evolution at higher field with a frequency of 2.5 kHz at 54 mu T. The latter should be taken into consideration for an efficient alt-SABRE. Theoretical and experimental findings were exemplified with Iridium N-heterocyclic carbene SABRE complex and N-15-acetonitrole, where a 30 % higher N-15 polarization with alt-SABRE compared to common SABRE was reached.