Figure 6.

Possible polarization transfer mechanisms to spin-1/2 heteronuclei of pyridine derivatives. (a) Indirect polarization transfer: the polarization of hydrogen is transferred to hydrogen of the pyridine derivative and subsequently to the corresponding heteronucleus X, e.g ¹⁹F. (b) Direct polarization transfer: the polarization is transferred directly to the corresponding heteronucleus X of the exchangeable ligand. (c) Polarization transfer after H/H exchange reaction. Hydrogens of a pyridine ligand in ortho-position can exchange during SABRE reaction⁵⁶ which would also enable a direct polarization transfer from polarized hydrogen atoms (to the corresponding heteronucleus of the exchangeable ligand X (here ¹⁹F).