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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Since the discovery of the CIDNP phenomenon,['. nuclear spin polarization effects in NMR spectra of chemical reactions have traditionally been ascribed to radical intermed i a t e~.[~] Bowers and Weitekarnpr4-51 showed, however, that very similar phenomena can also be observed in chemical reactions involving spin-polarized hydrogen. Thus, the cause of a nuclear spin polarization found by N M R spectroscopy is not unambiguous. Indeed, NMR polarization signals have been observed in the past during hydrogenation in the presence of organometallic catalysts[61 and often wrongly interpreted as CIDNP effects.['] Here we describe the experimental elucidation of the cause of nuclear spin polarization by a method involving the two isomers of molecular hydrogen (i.e., ortho and para hydrogen).So Par, nuclear spin polarizations during hydrogenations have been investigated exclusively by using para hydrogen, as reflected in the acronyms PASADENA (Parahydrogen And Synthesis Allows Dynamically Enhanced Nuclear Alignment) ['] or PHIP (Para Hydrogen Induced Polarization). '8,9] We have now studied these effects with both ortho and para hydrogen. ["] Molecular hydrogen contains two protons having nuclear spin I = 1/2. These spins couple to give a total spin 1 (tripletstate nuclear spin in ortho hydrogen) or 0 (singlet-state nuclear spin in para hydrogen). At room temperature, the small energy difference between the two spin isomers can be neglected. Owing to the threefold degeneracy of the triplet state, the ratio of the two isomers at room temperature is 75% ortho to 25% para. Hydrogenation with H, in this ratio leads to intermediates and final products whose N M R signals are characterized by the slight difference in thermal occupation (ANIN z lo-') of the nuclear spin states in the magnetic field.If, on the other hand, only one of the two spin isomers--for example, para hydrogen-is used in the hydrogenation, then, in the intermediates and final products, the only nuclear spin states occupied are those exhibiting singlet character with respect to the protons undergoing transfer. All other states remain unoccupied. This enormous difference in occupation leads to strong emission and absorption signals in the N M R spectrum.The mixture can be enriched in ortho hydrogen by exploiting the fact that, at low temperatures, the two spin isomers are not equally adsorbed on surfaces (theory of the hindered rotator["]). Gas chromatography on alumina as a diamagnetic adsorber at 77 K thus affords a mixture enriched in ortho hydrogen." This method yields a mixture containing about 80% ortho and 20% para hydrogen.A mixture enriched in para hydrogen can be obtained by making use of the fact that, in the presence of a paramagnetic adsorber (active charcoal), a thermal equilibrium is established between ortho and para hydrogen. At low temperatures, therefore, para hydrogen is favored In this way, we obtained a mixture of approximately 46% ortho and 54% para hydrogen at 77 K.The isomer ratio in the mixture was determined by difference measurem...

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