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Abstract

Introduction One of the most promising signal enhancement techniques with respect to future biomedical applications is parahydrogen-based nuclear spin hyperpolarization.[1] In particular, the method Signal Amplification by Reversible Exchange (SABRE) allows for reproducible hyperpolarization.[2] Here, we investigate the effect of solvent on SABRE. Aims The solvent and co-solvent dependency of hyperpolarization is an important aspect for understanding the mechanism and estimation of achievable signal enhancement. We have an aim to find a most suitable solvent for hyperpolarization and subsequent two-phase separation of hyperpolarized media. Methods ¹H and ¹⁹F hyperpolarization were performed on 3-fluoropyridine. Acquisition was on a Bruker wide bore 7T NMR spectrometer. 2mL solvent (methanol, chloroform, toluene, or xylene) were added into a 10mm NMR tube. Around 1.7mg of non-activated Ir-IMes complex was dissolved with 4μL of 3- fluoropyridine. Both catalyst activation and ¹⁹F signal amplification were investigated, using 50% enriched parahydrogen (para-H₂) at 6 bar pressure shook with the sample for 10s at 6mT. Results Hyperpolarization in methanol, chloroform and toluene can already be detected after the first introduction of para-H₂. Solvent molecules interacting with the Ir cation influence the coupling system between 3-fluoropyridine and hydrides which leads to a positive phase of the enhanced ¹⁹F signal. In pure xylene no signal enhancement was detected. Directly after addition of 500μL methanol to xylene solution, no hyperpolarization was observable. However, after waiting several hours a clear ¹⁹F signal enhancement with negative phase was measured. Conclusions The solvent has a significant impact on SABRE reactions. Phase and enhancement of the detected signal depend on the specific coupling system. Steric reasons prevent direct hyperpolarization in pure xylene.