NUCLEAR-MAGNETIC-RESONANCE STUDIES OF HOMOGENEOUS CATALYSIS USING PARAHYDROGEN - ANALYSIS OF NUCLEAR SINGLET TRIPLET MIXING AS A DIAGNOSTIC-TOOL TO CHARACTERIZE INTERMEDIATES

The H-1-NMR polarization spectra of the products after homogeneous hyd rogenations with para- or orthohydrogen (the PHIP spectra) convey sign ificant information about the course of the reaction. Mixing of the nu clear singlet and triplet states of the two protons derived from dihyd rogen during the reaction cycle has been found to modify the PHIP sign al pattern of the product molecule characteristically. The intermediat e coordination of the two dihydrogen protons at the catalyst is propos ed to be responsible for this behavior. A parallel to the singlet-trip let mixing of electronic states according to the well-known CIDNP phen omenon is outlined. The homogeneous hydrogenation of 2,5-dihydrofuran to tetrahydrofuran using Wilkinson's catalyst serves as an example.