Theoretical description of hyperpolarization formation in the SABRE-relay method

Abstract

SABRE (Signal Amplification By Reversible Exchange) has become a widely used method for hyperpolarizing nuclear spins, thereby enhancing their Nuclear Magnetic Resonance (NMR) signals by orders of magnitude. In SABRE experiments, non-equilibrium spin order is transferred from parahydrogen to a substrate in a transient organometallic complex. Applicability of SABRE is expanded by the methodology of SABRE-relay, in which polarization can be relayed to a second substrate either by direct chemical exchange of hyperpolarized nuclei or by polarization transfer between two substrates in a second organometallic complex. To understand the mechanism of the polarization transfer and study the transfer efficiency, we propose a theoretical approach to SABRE-relay, which can treat both spin dynamics and chemical kinetics as well as the interplay between them. The approach is based on a set of equations for the spin density matrices of the spin systems involved (i.e., SABRE substrates and complexes), which can be solved numerically. Using this method, we perform a detailed study of polarization formation and analyse in detail the dependence of attainable polarization level on various chemical kinetic and spin dynamic parameters. We foresee applications of the present approach for optimizing SABRE-relay experiments with the ultimate goal of achieving maximal NMR signal enhancements for substrates of interest.

Relayed Hyperpolarization for Zero-Field Nuclear Magnetic Resonance

“…The investigated region up to 25 mT revealed the presence of two maxima (Figure 1D). The first peak at 6.5 mT is the expected maximum corresponding to the symmetric active SABRE complex [Ir(IMes)(BnNH2)3H2], Figure 1A, and the conventional polarization-transfer mechanism as described above (45). The nature of the second peak is still under investigation, but may have two possible explanations.…”

“…It was reported that the presence of water detrimentally effects SABRE-relay efficiency (34). This is possibly due to the accelerated proton exchange which may affect protons in the carrier amine during the time it is bound to the complex, thus, altering the spin dynamics (45). Our findings in the conditions studied show that 1 H NMR signal enhancement of methanol is decreased by a factor of ~2 after addition of 5 µL of water to 0.5 mL of the SABRE-relay sample with BT ~ 6 mT (Figure S4).…”

Recent advances in the application of parahydrogen in catalysis and biochemistry

“…This transfer can occur either as a direct process via the scalar coupling connecting the two nuclei or as a relayed process, where the polarization is transferred in steps: first to nearby and stronger coupled nuclei and then to the nuclei of interest. A theoretical approach to SABRE-relay, which can treat both spin dynamics and chemical kinetics as well as the interplay between them, and allows optimization of SABRE-relay experiments with the ultimate goal of achieving maximal NMR signal enhancements for substrates of interest, was developed by Ivanov et al 119 …”

“…Due to its simplicity and ability to unravel unobservable chemical species, the utilized CEST NMR approach has a large application potential for studying short-lived hydride intermediates in catalytic reactions. To model such phenomena, Knecht, Ivanov et al 119 developed techniques for simulating the spin dynamics in the presence of chemical exchange. These techniques allowed them to describe the formation of hyperpolarized o-H 2 and to model quantitatively the PANEL experiment.…”

“…These techniques allowed them to describe the formation of hyperpolarized o-H 2 and to model quantitatively the PANEL experiment. Employing these techniques, it was possible to describe the polarization formation in SABRE-relay, an extension of the traditional SABRE for polarizing a wider range of substrates by exploiting additional exchange processes (see Knecht et al 119 for details). These techniques were later extended to treat highly complex processes with p-H 2 , involving several exchange pathways.…”