NMR Study of Nuclear Spin Polarization during Chemical Reactions with Ortho Hydrogen

Abstract

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...

Parahydrogen‐Based Hyperpolarization for Biomedicine

“…[77] An extension to other immobilized complexes (Rh, Ir, Au) followed, [78] but most of these catalysts demonstrate insufficient stability and activity. HET-PHIP effects were also successfully observed using heterogeneous catalysts comprising Pt and Pd nanoparticles supported on γ-Al 2 O 3 , [79] and later for many other supported metals (e.g., Rh, Ir, Co, Ni, Au, Cu) catalysts in gas, liquid and aqueous phase hydrogenations. [11–12, 25, 27, 78f, 78g, 80] Variation of the type of metal, metal particle size [80a, 80d, 81] and shape, [80e, 80h] type of support [80a, 80d, 80g, 80i] and reaction conditions were all shown to influence the magnitude of the PHIP effect.…”

In situ NMR Methods in Catalysis

“…The prerequisites for observing spin hyperpolarization in the NMR spectra acquired during or after hydrogenation are: (a) The two hydrogen atoms must be transferred in pairs. Similar but inverted NMR signal patterns are achieved with H 2 enriched in its nuclear triplet state (i.e., enriched in orthohydrogen) [88]. (c) The symmetry of the transferred hydrogen atoms must be broken during the reaction.…”

Hyperpolarized NMR Spectroscopy: d‐DNP, PHIP, and SABRE Techniques

“…This effect was first demonstrated by Bowers and Weitekampi nt he 1980s with the use of transition metal complexes as homogeneous hydrogenation catalysts, [37][38][39] and the acronym parahydrogen induced polarization (PHIP) [40,41] was later coined to describe the entire field of experimentsi nwhich the resultingh yperpolarized (HP) state is produced through ac hemical transformation of parahydrogen (pH 2 )g as. WhilstP HIP was used to study chemicalr eactions in the first 15 years after its invention, [41][42][43] it was not until the 2000s that the groups of Goldman, [44][45][46][47] Golman, [48] and others [49][50][51] pioneered approaches for intramolecular polarization transfer through spin-spin couplings from nascentp arahydrogen protons to significantly longer-lived hyperpolarized 13 Cc arboxyl sites (Figure 2a). Moreover,b ecause the in vivo 13 Cb ackgrounds ignal is low,H P 13 Cc ompounds were very quickly shown to be useful as in vivo contrast agents.…”