Comparing the S-T 0 Nuclear Spin Conversion in the Intermediates of Homogeneous and Heterogeneous Hydrogenations with Parahydrogen

In this work we present an initial attempt to study the S-T0 nuclear spin conversion in the pair of p-H2-derived protons within the hydrogenation intermediates under heterogeneous conditions. The results differ significantly from those obtained under homogeneous conditions where the S-T0 conversion demonstrated significant sensitivity to external factors, including temperature, solvent and catalyst. The singlet character of the spin state of the p-H2-derived protons transferred to the product molecule is also increased if a strong CW irradiation is applied at the anticipated chemical shift of the hydrogenation intermediates during p-H2 bubbling. In contrast, under heterogeneous hydrogenation conditions there is no or only a subtle dependence of the apparent S-T0 conversion in the hydrogenation intermediates on temperature and other external factors for all the catalytic systems tested. Different catalytic systems also demonstrated different results in terms of S-T0 conversion in the hydrogenation intermediates which could be related to the different magnetic environment and/or different lifetimes of the intermediates.