Peer-reviewed literature demonstrates that nuclear spin isomers, such as parahydrogen, engage in distinct chemical reactions and form transient complexes in processes like SABRE hyperpolarization.
A body of accumulated knowledge on the magneto-chemistry of catalysts for the hydrogen ortho-para conversion is reviewed in order to bridge the gap between theoretical work, laboratory experiments and current industrial needs. Some key issues raised by the conversion of hydrogen nuclear spin isomers, such as its magnetic patterns, which are only partially resolved, are first summarized and the industrial challenges currently faced regarding hydrogen liquefaction and storage are discussed. The theoretical analysis required to understand the quantum characteristics of hydrogen molecules begins with the thermodynamic properties of non-equilibrium mixtures of their two varieties, ortho and para. This is followed by a description of the various electro-magnetic catalytic channels linked to the specific properties of the catalysts, and a study of the energy and momenta exchanges between the catalysts and hydrogen gas. The third section is devoted to the magnetochemistry of catalysts, their morphologies, surfaces and porous structures, their ability to diffuse or adsorb a flow of hydrogen, and measurements of catalytic rates, with particular emphasis on "<i>in situ</i>" and "site-specific" experimental methods. The fourth section presents current industrial challenges, highlighting the importance of catalytic steps, and their modeling, in hydrogen liquefaction and storage. The irreversible nature of hydrogen flow through porous catalysts in liquefiers is discussed, and an original short-term hydrogen reservoir model is presented. It is based on the cryogenic, cybernetic, hydrodynamic and catalytic properties of a barrage system of successive microporous catalysts. Finally, break-even times for storage processes are introduced and discussed.
In this review, we present the physical principles of the SABRE (Signal Amplification By Reversible Exchange) method. SABRE is a promising hyperpolarization technique that enhances NMR signals by transferring spin order from parahydrogen (an isomer of the H2 molecule that is in a singlet nuclear spin state) to a substrate that is to be polarized. Spin order transfer takes place in a transient organometallic complex which binds both parahydrogen and substrate molecules; after dissociation of the SABRE complex, free hyperpolarized substrate molecules are accumulated in solution. An advantage of this method is that the substrate is not modified chemically, and its polarization can be regenerated multiple times by bubbling fresh parahydrogen through the solution. Thus, SABRE requires two key ingredients: (i) polarization transfer and (ii) chemical exchange of both parahydrogen and substrate. While there are several excellent reviews on applications of SABRE, the background of the method is discussed less frequently. In this review we aim to explain in detail how SABRE hyperpolarization is formed, focusing on key aspects of both spin dynamics and chemical kinetics, as well as on the interplay between them. Hence, we first cover the known spin order transfer methods applicable to SABRE - cross-relaxation, coherent spin mixing at avoided level crossings, and coherence transfer - and discuss their practical implementation for obtaining SABRE polarization in the most efficient way. Second, we introduce and explain the principle of SABRE hyperpolarization techniques that operate at ultralow (<1 μT), at low (1μT to 0.1 T) and at high (>0.1 T) magnetic fields. Finally, chemical aspects of SABRE are discussed in detail, including chemical systems that are amenable to SABRE and the exchange processes that are required for polarization formation. A theoretical treatment of the spin dynamics and their interplay with chemical kinetics is also presented. This review outlines known aspects of SABRE and provides guidelines for the design of new SABRE experiments, with the goal of solving practical problems of enhancing weak NMR signals.
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