Identifying routes for transferring spin polarization from parahydrogen to protic solvents.

Repeatable hyperpolarization of high concentrations of mobile protons (>6 M) using parahydrogen in protic methanol/water mixtures is reported here. Different ammonium buffers with increasing mobile proton concentrations were added to an IrCl(COD)(IMes) catalyst in the presence of pyridine. We reach a maximum molar polarization of 1.79 mM at 6 mT. Field-cycling experiments in an 18.8 T detection field distinguished two solvent polarization transfer pathways: chemical exchange with labile protons from ammonia and cross-relaxation with pyridine aromatic protons.