Abstract
We recently reported a pump-probe method that uses a single laser pulse to introduce parahydrogen (p-H2) into a metal dihydride complex and then follows the time-evolution of the p-H2-derived nuclear spin states by NMR. We present here a theoretical framework to describe the oscillatory behaviour of the resultant hyperpolarised NMR signals using a product operator formalism. We consider the cases where the p-H2-derived protons form part of an AX, AXY, AXYZ or AA′XX′ spin system in the product molecule. We use this framework to predict the patterns for 2D pump-probe NMR spectra, where the indirect dimension represents the evolution during the pump-probe delay and the positions of the cross-peaks depend on the difference in chemical shift of the p-H2-derived protons and the difference in their couplings to other nuclei. The evolution of the NMR signals of the p-H2-derived protons, as well as the transfer of hyperpolarisation to other NMR-active nuclei in the product, is described. The theoretical framework is tested experimentally for a set of ruthenium dihydride complexes representing the different spin systems. Theoretical predictions and experimental results agree to within experimental error for all features of the hyperpolarised 1H and 31P pump-probe NMR spectra. Thus we establish the laser pump, NMR probe approach as a robust way to directly observe and quantitatively analyse the coherent evolution of p-H2-derived spin order over micro-to-millisecond timescales.
Highlights
Hyperpolarisation is an increasingly important area of development in magnetic resonance, in fields such as biomedical NMR and MRI, where applications are often sensitivity limited.[1]
NMR signal enhancements can be observed for other NMR-active nuclei in the product molecule due to spontaneous or radio-frequency driven polarisation transfer from the protons originally on p-H2.[10, 13,14,15,16,17,18] This polarisation transfer phenomenon is harnessed in the signal amplification by reversible exchange (SABRE) approach, which uses a reversible exchange reaction to catalytically transfer polarisation from p-H2-derived protons to nuclei in a target substrate molecule without chemical alteration of the substrate.[10, 19]
We have chosen to work with ruthenium dihydride complexes, where the rates of H2 addition (Scheme 1) at 3 bar H2 are expected to be on the order of k = 106 s-1 on the basis of flash photolysis measurements,[31, 81,82,83] several orders of magnitude faster than the corresponding hydride chemical shift/J coupling differences
Summary
Hyperpolarisation is an increasingly important area of development in magnetic resonance, in fields such as biomedical NMR and MRI, where applications are often sensitivity limited.[1]. The NMR signals of the former p-H2-nuclei in the product can be enhanced by several orders of magnitude.[7, 11, 12] In some cases, NMR signal enhancements can be observed for other NMR-active nuclei in the product molecule due to spontaneous or radio-frequency (rf) driven polarisation transfer from the protons originally on p-H2.[10, 13,14,15,16,17,18] This polarisation transfer phenomenon is harnessed in the signal amplification by reversible exchange (SABRE) approach, which uses a reversible exchange reaction to catalytically transfer polarisation from p-H2-derived protons to nuclei in a target substrate molecule without chemical alteration of the substrate.[10, 19] Parahydrogen is a attractive source of hyperpolarisation because it is relatively cheap and easy to produce by cooling H2 gas in the presence of a paramagnetic species (e.g. activated charcoal or iron oxide). Once generated at low temperature (typically between 20 – 77 K depending on the desired level of p-H2 enrichment) p-H2 can be stored at room temperature for weeks or even months.[12]
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