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The 1H–NMR polarization spectra of the products after homogeneous hydrogenations with para- or orthohydrogen (the PHIP spectra) convey significant information about the course of the reaction. Mixing of the nuclear singlet and triplet states of the two protons derived from dihydrogen during the reaction cycle has been found to modify the PHIP signal pattern of the product molecule characteristically. The intermediate coordination of the two dihydrogen protons at the catalyst is proposed to be responsible for this behavior. A parallel to the singlet–triplet mixing of electronic states according to the well-known CIDNP phenomenon is outlined. The homogeneous hydrogenation of 2,5-dihydrofuran to tetrahydrofuran using Wilkinson’s catalyst serves as an example.

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