Second, it is inexpensive and commercially available. Third, it is non-toxic and relatively safe to use in a general laboratory setting. Finally, it is well known that ACN binds to the Ir(IMes) catalyst and 15 N SABRE polarization of ACN of up to 20% have been reported in the literature with moderate stability for low concentrations (≤ 100 mM). Therefore, a better understanding of the chemical processes that take place during the activation could improve the polarization values and stability of the SABRE system. Another advantage of this system concerns the polarizability of the naturally abundant N spins through SABRE-SHEATH (SABRE in SHield Enables Alignment Transfer to Heteronuclei). Instead of polarizing all molecules in the solvent, only "dilute" [ 15 N]-ACN isotopomers form an effective three-spin system in PTCs. The 14 N spins are effectively self-decoupled due to their accelerated quadrupolar relaxation, which minimizes polarization losses to 14 N-ACN isotopomers. This ensures that the polarization transfer is directed toward the [ 15 N]-ACN isotopomers, allowing them to be efficiently polarized.