parahydrogen Induced Polarization by Homogeneous Catalysis: Theory and Applications

Abstract

The alignment of the nuclear spins in parahydrogen can be transferred to other molecules by a homogeneously catalyzed hydrogenation reaction resulting in dramatically enhanced NMR signals. In this chapter we introduce the involved theoretical concepts by two different approaches: the well known, intuitive population approach and the more complex but more complete density operator formalism. Furthermore, we present two interesting applications of PHIP employing homogeneous catalysis. The first demonstrates the feasibility of using PHIP hyperpolarized molecules as contrast agents in (1)H MRI. The contrast arises from the J-coupling induced rephasing of the NMR signal of molecules hyperpolarized via PHIP. It allows for the discrimination of a small amount of hyperpolarized molecules from a large background signal and may open up unprecedented opportunities to use the standard MRI nucleus (1)H for, e.g., metabolic imaging in the future. The second application shows the possibility of continuously producing hyperpolarization via PHIP by employing hollow fiber membranes. The continuous generation of hyperpolarization can overcome the problem of fast relaxation times inherent in all hyperpolarization techniques employed in liquid-state NMR. It allows, for instance, the recording of a reliable 2D spectrum much faster than performing the same experiment with thermally polarized protons. The membrane technique can be straightforwardly extended to produce a continuous flow of a hyperpolarized liquid for MRI enabling important applications in natural sciences and medicine.

Parahydrogen discriminated PHIP at low magnetic fields

“…This yields the PHIP density operator at the end of the reaction ρ PH (0) (further details of the simulation method can be found in refs. [15,39]). …”

Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts

“…However if p-H 2 -derived hydrogen atoms are added to an unsaturated asymmetric substrate as a pair, the corresponding NMR signals of the reaction product are enhanced because the parahydrogen-derived protons retain their spin correlation. This effect was demonstrated with homogeneous 19,22 and heterogeneous 23–27 hydrogenation catalysts, the latter of which were utilized both in the liquid 25–27 and in the gas 23,24 phases. Applied to heterogeneous catalysis, PHIP provides very valuable mechanistic 28–32 and kinetic 33 insights and can be used for MRI visualization of operating catalytic reactors.…”

Parahydrogen-Induced Hyperpolarization inside Meso- and Micropores of Pt-, Rh-, Ir-, and Pd-Containing Solid Catalysts

“…4,5 In the past decade, PHIP demonstrated a high potential for the investigation of mechanisms of numerous homogeneously catalyzed hydrogenation reactions. [6][7][8][9][10][11][12][13] A very modern field for the application of PHIP is the increase of the NMR detection limit for studies of biological systems and for imaging techniques suitable for applications in medicine. [13][14][15][16] Very recently, first in situ continuous-flow MAS NMR spectroscopic studies of the formation of PHIP on noble metal-containing solids were performed with the aim to develop novel experimental approaches for mechanistic studies in the field of heterogeneous catalysis.…”