Efficient polarization redistribution in hyperpolarized 1-D-propane produced via pairwise parahydrogen addition

Abstract

Parahydrogen-Induced Polarization (PHIP) is NMR hyperpolarization technique that has matured from fundamental science to a biomedical tool for production of hyperpolarized MRI contrast agents. The spin order of nascent parahydrogen-derived protons can be employed directly for enhancement of their NMR signals or for polarization transfer to other nuclei in the hydrogenation product. In this work, we study the process of pairwise parahydrogen addition to propylene, which results in symmetric propane molecule with substantially enhanced methyl and methylene NMR signals. Specifically, we have synthesized site-selectively isotopically labeled 3-d-propylene molecule to study polarization dynamics in the resulting monodeuterated propane after pairwise parahydrogen addition. The deuterium presence in the hyperpolarized propane product results in a minute isotope chemical shift effect allowing to distinguish the proton resonances of CH₃ and CH₂D groups at 600 MHz. Pairwise parahydrogen 1,2-addition to 3-d-propylene was first confirmed by performing the reaction inside a 600 MHz NMR spectrometer, i.e., in the weakly-coupled regime at 14 T, where proton polarization dynamics is restricted to the molecular sites of parahydrogen addition. However, when the pairwise parahydrogen addition is performed in the strongly-coupled regime, i.e., at the Earth's magnetic field, efficient polarization transfer to CH₂D protons is readily observed, leading to polarization redistribution between the three inequivalent sites. This finding is important as it sheds light on polarization dynamics in the strongly coupled symmetric spin systems such as propane studied here—the presented results are expected to be applicable to other spin systems such as butane.