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 (15)N molecules. Here, we analyzed a variant of SABRE, consisting of a magnetic field alternating between a low field of ∼1 μT, where polarization transfer is expected to take place, and a higher field >50 μT (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 μT. 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 (15)N‐acetonitrole, where a 30 % higher (15)N polarization with alt‐SABRE compared to common SABRE was reached.