Isotopological Fingerprinting Using H/D Scrambling Identifies the Stereochemistry of Hyperpolarization Catalysts Transferring Spin Polarization from Parahydrogen to Substrates Using Signal Amplification by Reversible Exchange.

Hyperpolarization using signal amplification by reversible exchange (SABRE) relies on target molecules and parahydrogen coordinating to a transition metal catalyst. Identification of this coordinated state becomes increasingly important, especially since bio-relevant targets such as pyruvate and amino acids exhibiting multiple binding sites are becoming compatible with SABRE. In this report, we present a fingerprinting method to discriminate and identify ligand binding sites without requiring the presence of a sensitive or isotope-labeled heteroatom. Adding a small concentration of protons to a deuterated medium, spontaneous H/D scrambling of exchangeable protons encodes the ligands each with an isotopological fingerprint. By use of rapid 2D zero quantum NMR, the binding sites are decoded from the hydrides in less than a minute. The new methodology is explained and demonstrated on Ir mixed complexes with pyridine, benzylamine, and ammonia as common N-functional ligands.