About long-lived spin states involved in para-hydrogenated molecules

This study deals with a spin system constituted of three non-equivalent protons, two of which originating from para-hydrogen (p-H2) after a hydrogenation reaction carried out in the earth magnetic field. It is shown that i) three singlet states are created provided indirect (J) couplings exist between the three spins, implying hyperpolarization transfer toward the third spin; ii) these singlet states are converted into longitudinal orders upon insertion of the sample in the NMR (Nuclear Magnetic Resonance) high field magnet; iii) partial transfers further occurs between these longitudinal orders and polarizations. Nuclear spin relaxation is studied by appropriate NMR measurements when evolution takes place in the high field magnet or in the earth field. In the former case, relaxation is classical although complicated by numerous relaxation rates associated with both longitudinal orders and polarizations. In the latter case, an equilibration between the singlet states first occur, their disappearance being thereafter driven by relaxation rates which remain very small because of the absence of any dipolar contribution. Thus, even in the case of a three spin system, long-lived states exist; this unexpected property could be very useful for many applications.