⁷⁷Se Hyperpolarization Enabled by Reversible Parahydrogen Exchange and Audio-Frequency Magnetic Fields at 0.1 mT

Nuclear spin hyperpolarization boosts NMR sensitivity but typically requires complex infrastructure. Signal Amplification by Reversible Exchange (SABRE), a parahydrogen-based hyperpolarization technique, can be conducted using a remarkably low-cost polarizer (<$100) comprising only magnetic coils and a standard PC sound card, avoiding magnetic shielding. However, under these conditions, SABRE requires direct spin-coupling between parahydrogen and the target nucleus, limiting its application to distant heteronuclear spins such as ⁷⁷Se in biologically relevant selenium-nitrogen heterocycles (e.g., antioxidant, antiviral, and anticancer agents). Here, we overcome the distance restrictions by developing ¹⁵N-mediated polarization transfer driven by oscillating magnetic fields of the audio frequency. As a proof-of-concept, we achieve a 11600-fold enhancement of the ⁷⁷Se NMR signal (>6% net polarization) at 9.4 T in a shielding-free setup using a selenadiazole derivative. These results suggest that selenadiazole-based compounds are particularly well suited for this strategy and provide a basis for extending it to other selenium-containing molecules.