Manipulating Stereoselectivity of Parahydrogen Addition to Acetylene to Unravel Interconversion of Ethylene Nuclear Spin Isomers

Symmetric molecules exist as distinct nuclear spin isomers (NSIMs). A deeper understanding of their properties, including interconversion, requires efficient techniques for NSIMs enrichment. Selective hydrogenation of acetylene with parahydrogen (p-H2) was used to achieve the enrichment of ethylene NSIMs and to study their equilibration processes. The effect of stereoselectivity of H2 addition to acetylene on the imbalance of ethylene NSIMs was experimentally demonstrated by using different heterogeneous catalysts (an immobilized Ir complex and two supported Pd catalysts). The interconversion of NSIMs with time during ethylene storage was studied with NMR spectroscopy by reacting ethylene with bromine water which renders the p-H2-derived protons in the produced 2-bromoethan(2H)ol (BrEtOD) magnetically inequivalent, thereby revealing the non-equilibrium nuclear spin order of ethylene. A thorough analysis of the shape and transformation of the 1H NMR spectra of hyperpolarized BrEtOD allowed us to reveal the initial distribution of produced ethylene NSIMs and their equilibration processes. Comparison of the results obtained with different catalysts was key to properly attributing the derived characteristic time constants to different NSIMs interconversion processes: ~ 3-6 s for interconversion between NSIMs with the same inversion symmetry (i.e., within g or u manifolds) and ~ 1700-2200 s between NSIMs with different inversion symmetries.