Figure 4.

(a) Schematic depiction of a singlet order spin population for an inequivalent pair of coupled nuclear spins after hyperpolarization transfer in low field alongside the corresponding high-field populations after adiabatic transfer to high field. (b) Corresponding color-coded NMR spectral patterns predicted at high field after applying (i) a hard 90° flip angle pulse (purple, green, orange, and blue transitions in the above energy diagram) and (ii) a 10° flip angle pulse (purple and blue transitions). (c) ¹⁵N{¹H} NMR spectra of a CD₃OD reaction mixture containing the catalyst 1, diazirine 2, and pyr-d₅. The solution was photolyzed before the acquisition of every spectrum that was hyperpolarized by 20 s of bubbling with p-H₂. The pulse angle is increased from 10° to 170°, bottom to top. (d) SpinDynamica^47,48 simulated ¹⁵N{¹H} spectra for 4 assuming “pure” singlet order in the coupled ¹⁵N–¹⁵N system observed again by variable flip angle pulses of 10–170° in steps of 20° (bottom to top).