Characterisation of the polarisation transfer to fluorinated pyridines in SABRE

Signal amplification by reversible exchange (SABRE) is a fast and inexpensive hyperpolarisation method used to enhance NMR signals by several orders of magnitude. In this work, we focus on describing the mechanisms leading to hyperpolarisation in SABRE experiments on fluorinated pyridine derivatives. The polarisation transfer pathways to ligand fluorines and protons are explained by combining experimental results with spin dynamics simulations including chemical exchange and first-principles relaxation (Redfield theory). It is shown that ligand fluorines can be effectively hyperpolarised using two field regimes; in the μT range, the polarisation transfer can be attributed to coherent dynamics, but significant polarisation levels are also generated in the mT range through initial coherent polarisation of the ligand protons and the following incoherent processes conveying the polarisation to the fluorine nuclei. The role of different nuclei on the polarisation transfer was explored by simulations of spin systems of varied sizes, which revealed the importance of incorporating incoherent mechanisms consisting of the dipolar and quadrupolar relaxation as well as the scalar relaxation of the second kind, into the simulations, also at such magnetic fields where the coherent mechanism is mainly responsible for the polarisation transfer.