Although the use of a rotating-field STORM suppresses the deleterious effects of deuteron relaxation during the singlet-tomagnetization transfer, the problem of singlet-to-triplet mixing remains. In the current context of very low magnetic fields, this mixing is caused by differences in the J-couplings of the two protons to heteronuclei such as 13 C and 2 H (see the coupling network in figure ). As a result of these couplings, proton singlet state is not a Hamiltonian eigenstate for the succinic acid product. Reaction of the fumaric acid precursor with para-hydrogen therefore initiates coherent oscillations of the spin density operator. Since the hydrogenation reaction is relatively slow, these