Figure 4.

(a) ¹H NMR spectrum from a mixture containing d₆-ethanol solvent, the PVP polymer comb catalyst particles (PVPCs), and the (fully protonated) pyridine substrate thermally polarized at 9.4 T following activation with pH₂ bubbling. (b) ¹H NMR spectrum of the sample in (a) obtained after PVPC activation but with pH₂ bubbling occurring at high field, acquired immediately after cessation of pH₂ gas bubbling; for the most part, no high-field (in situ) SABRE effect was observed. (c) ¹H (ex situ) HET-SABRE NMR spectrum obtained from the same sample, acquired immediately after 30 s of pH₂ bubbling at low field (∼100 G) and rapid transfer of the sample into the NMR magnet. All spectra shown were acquired with a single scan (90° pulse). Peaks at about δ ≈1.1, ≈3.6, and ≈5.2 ppm are from residual protons from the deuterated ethanol solvent. The peak at ≈4.5 ppm is from (ortho-)hydrogen (oH₂) gas. The inset shows the expected hexacoordinate structure of the activated catalytic moiety, exchanging with pH₂ and the substrate pyridine (Py).