SYMMETRY BREAKDOWN DURING PARAHYDROGEN LABELING OF SYMMETRICAL SUBSTRATES - PROTON SPIN POLARIZATION AND SINGLET/TRIPLET MIXING DUE TO C-13-ISOTOPES IN NATURAL-ABUNDANCE

Proton spin polarization occurs not only during the hydrogenation of asymmetrically substituted acetylenes with parahydrogen (p-Hz) but also during the hydrogenation of symmetric hydrocarbon substrates: C-13-isotopes in natural abundance cause asymmetry even in seemingly symmetric products and break the magnetic symmetry of the former p-H-2 protons. The polarization patterns of the hydrogenated products sensitively depend on the degree of nuclear singlet/triplet (S/T-0) mixing in the intermediates of the catalytic reactions. Depending on the nature of the substrate, cationic catalysts hydrogenate substrates with different degrees of S/T-0 mixing, thereby yielding information about the catalytic reaction mechanisms. During the hydrogenation of symmetric diesters of acetylene dicarboxylic acid, unexpected polarization signals of products containing only C-12-isotopes have also been detected. These signals reflect an unusual nuclear spin population of yet unknown origin; their explanation demands an augmentation of the existing theoretical models.