Nuclear spin polarization

Closs G L 1969 A mechanism explaining nuclear spin polarizations In radical combination reactions J. Am. Chem. Soc. 91 4552-4... 

CHEMICALLY INDUCED DYNAMIC NUCLEAR SPIN POLARIZATION AND ITS APPLICATIONS... 

The first reports of the observation of transient emission and enhanced absorption signals in the H-n.m.r. spectra of solutions in which radical reactions were taking place appeared in 1967. The importance of the phenomenon, named Chemically Induced Dynamic Nuclear Spin Polarization (CIDNP), in radical chemistry was quickly recognized. Since that time, an explosive growth in the number of publications on the subject has occurred and CIDNP has been detected in H, C, N, and P as well as H-n.m.r. spectra. Nevertheless, the number of groups engaged in research in this area is comparatively small. This may be a consequence of the apparent complexity of the subject. It is the purpose of this review to describe in a quahtative way the origin of CIDNP and to survey the published applications of the phenomenon in... 

The lifetime of the radical pair is important since successive encounters of radicals [going between case (1) and case (2)] will give more Tq-S mixing and therefore greater nuclear spin polarization. 

Kuhn LT, Bargon J (2007) Exploiting Nuclear Spin Polarization to Investigate Free Radical Reactions via in situ NMR. 276 125-154... 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

Chain processes, free radical, in aliphatic systems involving an electron transfer reaction, 23,271 Charge density-NMR chemical shift correlation in organic ions, 11,125 Chemically induced dynamic nuclear spin polarization and its applications, 10, 53 Chemiluminescence of organic compounds, 18,187... 

The observed polarization is primarily associated with the former parahydrogen protons. However, other protons may also experience a drastic signal enhancement due to nuclear spin polarization transferred to these nuclei via the nuclear Overhauser effect (NOE) or similar processes, both in the final reaction products as well as in their precursor intermediates. 

The fact that two entirely different phenomena can both yield nuclear spin polarization may cause confusion therefore, the appearance of intense emission and absorption lines during in-situ NMR investigations of hydrogenation reactions is not necessarily proof for free radical intermediates, and examples of erroneous conclusions do exist [5]. 

Integrated thermal conductivity cells (see Fig. 12.8) allow a quantitative determination of the corresponding ortho/para ratios of the dihydrogen. The enriched parahydrogen is well-suited for in-situ NMR studies of hydrogenation reactions that yield nuclear spin polarization due to symmetry breaking during the reaction. The same apparatus has also been used successfully to enrich ortho- and paradeuterium mixtures. 

Chemically induced dynamic nuclear spin polarization and its applications, 10, 53... 

A third source of misassignment has its roots in the existence of nuclear-nuclear cross- relaxation." Again, depending on the mechanism of cross-relaxation and on the polarization of the originally polarized nucleus, this may result in enhanced absorption or emission. This process induces nuclear spin polarization in nuclei without hfc, or alters the nuclear spin polarization of nuclei with weak hfcs. On the other hand, the magnitude of these effects may be quite small and fall below the threshold of chemical significance. 

Finally, it should be pointed out that methods used to study short-lived chemical intermediates in fast thermal reactions may be applicable also to photochemical studies. Radical intermediates, however generated, can be studied by CIDNP ichemically induced dynamic nuclear spin polarization), in which the n.m.r. spectrum of the reaction mixture is recorded during the reaction period. II a substrate is continuously irradiated with ultraviolet/visible light in the cavity of an n.m.r. spectrometer, the resulting n.m.r. spectrum of the substrate/product mixture exhibits intensity variations as compared with the normal spectrum—intensity enhancement, reduction or even reversal (i.e. emissionl. Note that the spectrum involved is not... 

The isomerization of donor olefins is illustrated by the reaction of chloranil with a pair of geometric isomers, cis- and trans-1 -phenylpropene. The irradiation of the quinone in polar solvents in the presence of either isomer results in nuclear spin polarization for both isomers. The key to understanding these effects lies in two observations (Fig. 9) (a) the polarization of the regenerated parent olefin is stronger than that of the rearranged olefin (b) the reaction of the ds-isomer results in stronger overall effects than does that of the trans-isomer [157, 158],... 

ESR spectroscopy has found wide-spread use for the detection of radical intermediates in electrode processes 40 For the same purpose, the newly developed technique of trapping short-lived radicals by nitrones or nitroso compounds 40d-> should be of considerable interest, as should also the chemically induced nuclear spin polarization (CINP) phenomenon 40e-1 be. 

Kompitsas, M. and Weber, H.G. (1975). A study of the nuclear spin polarization of Na2 by optical pumping, Chem. Phys. Lett., 35, 277-279. 

Weber, H.G., Glass, H.-J., Huber, R., Kompitsas, M., Schmidt, G. and zu Putlitz, G. (1974). Optical pumping method for studying nuclear-spin polarization in alkali dimers, Z. Physik, 268, 91-95. 

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