Adiabatic Passage through Level Anticrossings in Systems of Chemically Inequivalent Protons Incorporating Parahydrogen: Theory, Experiment, and Prospective Applications

Magnetic field dependence of the para-ortho conversion rate of molecular hydrogen in SABRE experiments

The use of parahydrogen - the isomer of molecular hydrogen with zero nuclear spin - is important for promising and actively developing methods for spin hyperpolarization of nuclei called parahydrogen induced polarization (PHIP). However, the dissolved parahydrogen in PHIP experiments quickly loses its spin order, resulting in the formation of orthohydrogen and reduction of the overall nuclear polarization of the substrate. This process is due to the difference of chemical shifts of hydride protons, as well as spin-spin couplings between nuclei, in the intermediate catalytic complexes, and it has not been rigorously explained so far. We proposed a new experimental technique based on magnetic field cycling for measuring the rate of molecular hydrogen para-ortho conversion in solution and applied it for non-hydrogenative PHIP Signal Amplification By Reversible Exchange (SABRE) experiments. The para-ortho conversion rate was measured over a wide range of magnetic field from 0.5 mT to 9.4 T. It was found that the conversion rate strongly depends on the magnetic field in which the reaction occurs, as well as on the concentrations of reactants. The rate decreases with increasing the concentration of pyridine ligand and increases with increasing the concentration of iridium catalyst. The model, which takes into account the reversible exchange of molecular hydrogen with the catalyst, nuclear spin-spin interaction of hydride protons with nuclei of ligands within catalytic complex and nuclear Zeeman interactions, qualitatively describes the experimental data. Two types of complexes with different spin system symmetry contribute to the molecular hydrogen conversion. In asymmetric complexes possessing hydride protons with different chemical shifts due to the presence of chlorine anion ligand the para-ortho conversion rate increases with magnetic field, while for symmetric complexes this mechanism is not operable. In the magnetic field where level anti-crossing occurs the resonant feature for the rate of para-ortho conversion is found. The results of this work can be utilized for finding the optimal conditions for obtaining the maximum hyperpolarization in the experiments employing parahydrogen.

StereoPHIP: Stereoselective Parahydrogen-Induced Polarization

Parahydrogen-induced polarization (PHIP) followed by polarization transfer to 13 C is a rapidly developing technique for the generation of 13 C-hyperpolarized substrates. Chirality plays an essential role in living systems and differential metabolism of enantiomeric pairs of metabolic substrates is well documented. Inspired by asymmetric hydrogenation, here we report stereoPHIP, which involves the addition of parahydrogen to a prochiral substrate with a chiral catalyst followed by polarization transfer to 13 C spins. We demonstrate that parahydrogen could be rapidly added to the prochiral precursor to both enantiomers of lactic acid (D and L), with both the (R,R) and (S,S) enantiomers of a chiral rhodium(I) catalyst to afford highly 13 C-hyperpolarized (over 20 %) L- and D-lactate ester derivatives, respectively, with excellent stereoselectivity. We also show that the hyperpolarized 1 H signal decays obtained with the (R,R) and (S,S) catalysts were markedly different. StereoPHIP expands the scope of conventional PHIP to the production of 13 C hyperpolarized chiral substrates with high stereoselectivity.

Spying on parahydrogen-induced polarization transfer using a half-tesla benchtop MRI and hyperpolarized imaging enabled by automation

Nuclear spin hyperpolarization is a quantum effect that enhances the nuclear magnetic resonance signal by several orders of magnitude and has enabled real-time metabolic imaging in humans. However, the translation of hyperpolarization technology into routine use in laboratories and medical centers is hampered by the lack of portable, cost-effective polarizers that are not commercially available. Here, we present a portable, automated polarizer based on parahydrogen-induced hyperpolarization (PHIP) at an intermediate magnetic field of 0.5 T (achieved by permanent magnets). With a footprint of 1 m2, we demonstrate semi-continuous, fully automated 1H hyperpolarization of ethyl acetate-d6 and ethyl pyruvate-d6 to P = 14.4% and 16.2%, respectively, and a 13C polarization of 1-13C-ethyl pyruvate-d6 of P = 7%. The duty cycle for preparing a dose is no more than 1 min. To reveal the full potential of 1H hyperpolarization in an inhomogeneous magnetic field, we convert the anti-phase PHIP signals into in-phase peaks, thereby increasing the SNR by a factor of 5. Using a spin-echo approach allowed us to observe the evolution of spin order distribution in real time while conserving the expensive reagents for reaction monitoring, imaging and potential in vivo usage. This compact polarizer will allow us to pursue the translation of hyperpolarized MRI towards in vivo applications further.

Radio-Frequency Sweeps at Microtesla Fields for Parahydrogen-Induced Polarization of Biomolecules

Magnetic resonance imaging of ¹³C-labeled metabolites enhanced by parahydrogen-induced polarization (PHIP) enables real-time monitoring of processes within the body. We introduce a robust, easily implementable technique for transferring parahydrogen-derived singlet order into ¹³C magnetization using adiabatic radio frequency sweeps at microtesla fields. We experimentally demonstrate the applicability of this technique to several molecules, including some molecules relevant for metabolic imaging, where we show significant improvements in the achievable polarization, in some cases reaching above 60% nuclear spin polarization. Furthermore, we introduce a site-selective deuteration scheme, where deuterium is included in the coupling network of a pyruvate ester to enhance the efficiency of the polarization transfer. These improvements are enabled by the fact that the transfer protocol avoids relaxation induced by strongly coupled quadrupolar nuclei.

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies

The heterogeneous solid-gas reactions of crystals of [Rh(L₂)(propene)][BAr^F₄] (1, L₂ = ^tBu₂PCH₂CH₂P^tBu₂) with H₂ and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para-H₂ transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan ¹H, ¹³C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 1 reacts with H₂ to form dimeric [Rh(L₂)(H)(μ-H)]₂[BAr^F₄]₂ (4), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para-H₂ with its NMR active ortho isomer, contemporaneously converting into 4, and 1 and 4 each convert para-H₂ into ortho-H₂ at different rates. Hydrogenation of propene using 1 and para-H₂ results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1-butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh(^tBu₂PCH₂CH₂P^tBu₂)(1,3,4-Me₃C₆H₃)][BAr^F₄], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H₂ more slowly to form catalytically active 4. Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H₂ is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy₂PCH₂CH₂PCy₂ results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy₂PCH₂CH₂PCy₂){η⁶-(CF₃)₂(C₆H₃)}BAr^F₃].