Figure 3.

a–b) Conversion of ‘normal’ H₂ gas (75% ortho- and 25% para-isomers) into parahydrogen (p-H₂). a) Schematic of a p-H₂ generator, where the entering room-temperature (‘normal’) H₂ gas is cryo-cooled to ≤77 K and catalytically converted under local equilibrium to a mixture of spin isomers that preferentially favors p-H₂. Because the catalyst is confined to a cryogenically cooled chamber of the polarizer, once the p-H₂ leaves the chamber it is kinetically trapped in the para-state. b) Temperature dependence of the equilibrium p-H₂ percentage. Liquid N₂ temperature (77 K) allows for preparation of 50% p-H₂, whereas temperatures below 20 K enable production of >97% p-H₂ fraction. c) The process of Parahydrogen Induced Polarization (PHIP). The pairwise addition of a p-H₂ molecule to a molecular precursor, which is typically accomplished across a C=C or C≡C bond adjacent to a ¹³C nucleus using a hetero- or homogeneous catalyst. The resulting chemically ‘unlocked’ nuclear spin hyperpolarization of the nascent, magnetically inequivalent protons can be used as-is, or transferred to the typically longer-lived (greater T₁) ¹³C site using either a RF pulse sequence or a field-cycling method. d) The process of Signal Amplification by Reversible Exchange (SABRE). A metal complex [M] enables p-H₂ and a substrate to be transiently co-located under conditions of dynamic exchange, resulting in spontaneous polarization transfer^{[11, 62]} from p-H₂ to the substrate.