Figure 6.

Kinetic model for gem-PHIP. The chemical reaction of para-enriched ${\mathrm{H}}_{\mathrm{2}}$ with the acetylenedicarboxylate precursor and ruthenium-based catalyst (not shown) proceeds with rate constant $k$ . The shaded area includes the product molecule I in different spin polarization states: unpolarized (top), in a state of singlet nuclear spin order (left), in a state of $zz$ order (right), and with observable $x$ magnetization (bottom). Singlet order decays with rate constant $R_{\mathrm{S}}=T_{\text{S}}^{-\mathrm{1}}$ ; $zz$ order decays with rate constant $R_{zz}=T_{zz}^{-\mathrm{1}}$ . If no spin-locking field is present, singlet order is rapidly converted into $zz$ order, with a conversion factor of $-\mathrm{2}/\mathrm{3}$ (blue arrow and yellow box). If a spin-locking field is applied, $zz$ order is instantaneously projected onto singlet order, with a conversion factor of $-\mathrm{1}/\mathrm{2}$ (orange arrow and yellow box). Singlet order and $zz$ order may be converted into observable $x$ magnetization by the two-pulse sequence in Fig.  (red arrows and yellow boxes). The conversion factors in this case are $\mathrm{2}/\mathrm{3}$ for singlet order and $-\mathrm{1}/\mathrm{2}$ for $zz$ order.