Para-hydrogen-induced polarization PHIP

Other important topics, such as the use of para-hydrogen-induced polarization (PHIP) NMR, are discussed in more detail elsewhere in this book. Basically, this approach enhances the NMR signal one thousandfold, thus allowing the detection of intermediates that go unnoticed when using classicaF NMR techniques. PHIP is particularly suited for homogeneous hydrogenation research because a prerequisite of the method is that both former para-hydrogen nuclei must be present (and J-coupled) in the molecule of interest. 

This model has been used for the description of Arrhenius curves of H-transfers as described in more detail in Chapter 6. The next two chapters show applications of these symmetry effects. First the para-hydrogen induced polarization (PHIP) experiments are discussed. There the symmetry induced nuclear spin polarization creates very unconventional NMR lineshape patterns, which are of high diagnostic value for catalytic studies. Then in Section 21.4 symmetry effects on NMR line-shapes and relaxation data of intramolecular hydrogen exchange reactions are discussed and examples from iH-liquid state and H-solid state NMR are presented and compared to INS spectra. The last section gives an outlook on possible future developments in the field. 

In another class of experiments, hyperpolarized states are generated by spin-sensitive chemical reactions. These include para-hydrogen-induced polarization (PHIP) [3-5] and chemically induced dynamic nuclear polarization (CIDNP) [6-8]. The latter involves non-equilibrium nuclear spin state populations that are produced in chemical reactions that proceed through radical pair intermediates. CIDNP s applicability has been focused towards the study of chemical reactions and the detection of surface exposed residues in proteins [9], but has so far remained limited to specialized chemical systems. 

Another advanced NMR technique is para-hydrogen-induced polarization (PHIP) spectroscopy. Dihydrogen consists of two nuclear spin isomers, one of which has a total spin of zero (I = 0) and is called para-hydrogen. The other spin isomer has a total spin of one (/ = 1) and is called ortho-hydrogon. At room temperature, dihydrogen is a mixture of about 25% para- and 75% ortho-hydrogen. 

This concept has originally been named PASADENA (Parahydrogen And Synthesis Allow Dramatically Enhanced Nuclear Alignment) [6], but the spectroscopic method based on this phenomenon has subsequently also been called PHIP (Para-Hydrogen Induced Polarization) [7]. In this chapter the abbreviation PH IP will be used throughout. 

Finally, it has been shown how para-H2 effects in the NMR spectra of hydrogenated products are largely affected by the relaxation processes occurring at intermediate steps represented by hydride derivatives. It is reasonable to foresee that a better control of the hydride relaxation processes occurring at these intermediate steps will allow the detection of even more impressive effects and, as a consequence, novel applications of the PHIP (Para Hydrogen Induced Polarization) experiments. [49]... 

An alternative possibility to increase the sensitivity of solid-state NMR for the study of catalytic processes is the application of spin-polarized para-hydrogen. The most prominent example of this technique is the so-called Para Hydrogen Induced Polarization or short PHIP experiment, originally invented for liquid-state NMR studies of homogeneous catalysis. PHIP is not restricted to homogeneous hydration reactions and liquid state NMR, but is also applicable to studies of heterogeneous hydration... 

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