Spin dynamics in experiments on orthodeuterium induced polarization (ODIP)

A comprehensive description of the spin dynamics underlying the formation of Ortho-Deuterium Induced Polarization (ODIP) is presented. ODIP can serve as a tool for enhancing Nuclear Magnetic Resonance (NMR) signals of ²H nuclei, being important probes of molecular structure and dynamics. To produce ODIP, in the first step, the D₂ gas is brought to thermal equilibrium at low temperature, here 30 K, so that the ortho-component, corresponding to the total spin of the ²H nuclei equal to 0 and 2, is enriched, here to 92%. In the second step, the orthodeuterium molecule is attached to a substrate molecule using a suitable hydrogenation catalyst such that the symmetry of the two ²H nuclei is broken. As a result, the non-thermal spin order of orthodeuterium is converted into enhancement of observable NMR signals. In this work, we perform a theoretical study of ODIP and calculate the shape of ODIP spectra and their dependence on the magnetization flip angle. These results are compared with experiments performed for a number of substrates; good agreement between experimental and calculated ODIP spectra is found. We also discuss the performance of NMR techniques for converting anti-phase ODIP spectral patterns into in-phase patterns, which are more suitable for signal detection and for transferring ODIP to heteronuclei, here to ¹³C spins. Experimental procedures reported here allowed us to reach signal enhancement factors of more than 1000 for ²H nuclei in the liquid phase. These results are useful for extending the scope of spin hyperpolarization to the widely used ²H nuclei.