Harnessing p-H2 hyperpolarisation and heteronuclear couplings in Earth’s field NMR

The aim of this thesis was to explore the optimisation of PHIP (parahydrogen (p-H2) Induced Polarisation) for detection in the zero-to-ultra-low-field (ZULF) regime, specifically the Earth’s magnetic field (EF). First, I optimised the SABRE (Signal Amplification by Reversible Exchange) hyperpolarisation of three amines at 500 MHz using the pre-catalyst IrCl(IMes)(COD) (I) and a co-ligand, DMSO. This increased the 500 MHz polarisation level to 252-fold, which proved sufficient to enable EF detection. This was the first achievement of amines detection in the ZULF regime. In the case of 2,4,6-trifluoroamine, the heteronuclear 1H-19F coupling of ~5 Hz allowed differentiation between the hyperpolarised target and enhanced H2O signals, highlighting the benefits of heteronuclear couplings in the ZULF regime for chemical differentiation. Building on this, the potential for p-H2 hyperpolarisation of a series of 31P containing molecules with 1H-31P couplings ranging from ~15 – 600 Hz was investigated at high-field on reaction with I. Whilst several proved PHIP active, none were suitable for EF detection because of their slow p-H2 exchange. In order to overcome this, the phosphorus compounds were changed to a series of phosphines with aromatic (PR3) and cyclohexyl substituents. In view of earlier studies, the starting complex was changed to IrCl(DMSO)3. This complex proved to react with the aromatic phosphines to form an array of orthometallation products alongside some simple substitution products, IrCl(DMSO)y(PR3)x [where y = 0, 1 and 2; x = 3, 2, 1]. Under hydrogen, these complexes react to form [Ir(H)2(DMSO)(PR3)2Cl] and [Ir(H)2(DMSO)2(PR3)Cl] according to the phosphine. In the case of PPh3, the bis-phosphine product dominates and a PHIP (1 scan) in comparison to the thermal spectrum (32 scans) have signal-to-noise ratios of 5775 and 320, respectively. In contrast, when PCy3 was examined, the mono-phosphine product dominates, with an estimated polarisation level enhancement of 2000-fold at 11.7 T. Significant polarisation enhancements were also observed from the mono-phosphine product with PPh2PCy. The high levels of renewable 1H and 31P polarisation of these three phosphine adducts, combined with their large cis 31P – 1H couplings (~20 Hz), proved to make them ideal for detection using EF NMR. This is the firstly reported direct observation of hydride hyperpolarisation in the ZULF regime and was confirmed by comparison with density matrix simulations.