Hyperpolarization of 3-fluoropyridine via Signal Amplification by Reversible Exchange

Abstract

In nuclear magnetic resonance (NMR), inductive detection has been the only essential signal acquisition mechanism for decades. Recently, a fundamentally new way of detecting NMR, by observing nuclear spin-induced optical rotation (NSOR), was discovered. However, NSOR suffers from a poor signal-to-noise ratio (SNR), hindering its applications. One compelling way of improving the SNR by orders of magnitude is to use a relatively new hyperpolarization method called signal amplification by reversible exchange (SABRE).

The purpose of this thesis was to see whether SABRE could enable the NSOR signal detection of fluorine and to find ways to optimize an existing continuous-flow NSOR system. This involved studying the polarization transfer process in SABRE by performing several experiments with a 3-fluoropyridine sample.

The levels of obtained hyperpolarization were measured at different magnetic fields. Fluorine received the greatest levels of polarization at ultralow fields, whereas proton was hyperpolarized most effectively at milli-Tesla fields. This was attributed to the coherent polarization transfer at level anti-crossings (LACs). At magnetic field strengths where LACs are not present, the polarization transfer mechanism could not be unambiguously determined.

Measured relaxation times at different magnetic fields proved to be similar for fluorine and proton. At ultralow fields, the relaxation was rather quick (~ 3 s), whereas at low and high fields, slower relaxation was observed (~ 10 s). Therefore, having a well-defined magnetic field during the sample transfer would minimize the effects of relaxation and result in better levels of polarization.

Polarization build-up constants were measured at different magnetic fields. They were also quite comparable between the fluorine and the proton, while also being similar to the relaxation time constants. At ultralow fields, the polarization builds up fastest, which is a favourable result considering the continuous-flow NSOR system, as the sample is continuously pumped from the polarizing cell. Additionally, the fast build-up enables the use of faster pumping speeds to minimize the effects of relaxation during sample transfer.