Spin polarization induced nuclear Overhauser

Brunner, Pines and coworkers reported on the enhancement of NMR signals in solid Cgo and C70 using a laser-polarized xenon. NMR signals emanating from surface nuclei of solids may be enhanced by the transfer of spin polarization from laser-polarized noble gases via SPINOE (spin polarization induced nuclear Overhauser effect). The paper describes experiments in which the spin polarization is transferred under MAS from laser-polarized - Xe to a nuclear spin with a low gyromagnetic ratio in the fullerenes Ceo and C70, which are polycrystalline materials with a low surface area. In C70, a different degree of enhancement of the NMR spectrum is observed for the different atomic sites in the molecule. 

SPINOE Spin polarization induced nuclear Overhauser effect... 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

Methods of disturbing the Boltzmann distribution of nuclear spin states were known long before the phenomenon of CIDNP was recognized. All of these involve multiple resonance techniques (e.g. INDOR, the Nuclear Overhauser Effect) and all depend on spin-lattice relaxation processes for the development of polarization. The effect is referred to as dynamic nuclear polarization (DNP) (for a review, see Hausser and Stehlik, 1968). The observed changes in the intensity of lines in the n.m.r. spectrum are small, however, reflecting the small changes induced in the Boltzmann distribution. 

Several physical methods have been employed to ascertain the existence and nature of ICs infrared (IR) absorption spectroscopy nuclear magnetic resonance (NMR) spectroscopy,14 including JH nuclear Overhauser effect (NOE) difference spectroscopy, H 2-D rotating-frame Overhauser effect spectroscopy (2-D ROESY),15 and solid-state 13C cross-polarization/magic angle spinning (CP/MAS) spectroscopy 16 induced circular dichroism (ICD) absorption spectroscopy 17 powder and singlecrystal X-ray diffraction 18 and fast atom bombardment mass spectrometry (FAB MS). 

From the start, these phenomena were recognized as spin polarizations (deviations of the populations of the nuclear spin states from the Boltzmann distribution) caused by radical reactions. As the first attempts to understand their generation erroneously focussed on Overhauser effects, they were christened "chemically induced d)mamic nuclear polarizations". Although only partially correct, that name has stuck, possibly because its acronym CIDNP (usually pronounced "kidnap") evokes the picture of radical scavenging. However, only 2 years later the now universally accepted quite different explanation, the hitherto unknown radical-pair mechanism, was found, again by two groups independently." ... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

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