Nuclear polarization

Methods of disturbing the Boltzmann distribution of nuclear spin states were Imown 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, refiecting the small changes induced in the Boltzmann distribution. 

Although it is now established that CIDNP has a quite different origin from DNP, the two effects were initially thought to be related. Thus the Overhauser effect, in which saturation of unpaired electron spins leads to polarization of nuclei coupled to the electrons through the hyperfine coupling constant (a ), can be observed in organic radicals, and CIDNP 3 

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Lawler R G 1967 Chemically Induced dynamic nuclear polarization J. Am. Chem. Soc. 89 5519-21... 

Kaptein R and Oosterhoff J L 1969 Chemically Induced dynamic nuclear polarization III (anomalous multiplets of radical coupling and disproportionation products) Chem. Phys. Lett. 4 214-16... 

Goez M 1995 An introduction to chemically induced dynamic nuclear polarization Concepts Magn. Reson. 7 69-86... 

Kaptein R 1971 Simple rules for chemically induced dynamic nuclear polarization J. Chem. See. Chem. Commun. 732-3... 

Morozova O B, Tsentalovich Y P, Yurkovskaya A V and Sagdeev R Z 1998 Consecutive biradicals during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone low- and high-field chemically induced dynamic nuclear polarizations (CIDNP) study J. Rhys. Chem. A 102 3492-7... 

C3.2.2.12 COMPETITION BETWEEN INNER SPHERE AND OUTER SPHERE NUCLEAR POLARIZATION DYNAMICS... 

Analogies for such a mechanism in diazotization are found in the nitrous acid-catalyzed nitration of A,A-dimethylaniline, mesitylene, 4-nitrophenol, and some related compounds, which were investigated by 15N NMR spectroscopy in Ridd s group (Ridd and Sandall, 1981 Ridd et al., 1992 Clemens et al., 1984a, 1984b, 1985 Johnston et al., 1991 review Ridd, 1991). Ridd and coworkers were able to demonstrate clearly that not only the nitration proper, but also the preceding C-nitrosation, is accompanied by a marked 15N nuclear polarization. This was at-... 

Carboxamides, formation from ArNj 243 2-Carboxybenzenediazonium zwitterion, see 2-Diazoniobenzenecarboxylate Carboxy-de-diazoniation 241 f. Carcinogenicity 319 Chemically induced dynamic nuclear polarization, see CIDNP Chloro-de-diazoniation 194, 230ff., 247, 270, 277... 

CIDNP chemically induced dynamic nuclear polarization... 

For benzoyl and acetyl peroxides, loss of carbon dioxide occurs in a stepwise process. Estimates of the rate constants for step c in Scheme 1 are 7 x 10 sec (benzene, 60°). The corresponding process for acetyl peroxide has k = 2x 10 sec (n-hexane, 60°), so that the lifetime of radical pairs containing acetoxy radicals is comparable to the time necessary for nuclear polarization to take place (Kaptein, 1971b Kaptein and den Hollander, 1972 Kaptein et al., 1972). Propionoxy radicals are claimed to decarboxylate 15-20 times faster than acetoxy radicals (Dombchik, 1969). 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Kaptein, R. (1971b). Chemically Induced Dynamic Nuclear Polarization , Thesis, Leiden. 

Stevenson, S. and Dorn, H. C., 13C dynamic nuclear polarization a detector for continuous-flow, on-line chromatography, Anal. Chem., 66, 2993, 1994. 

Chemically Induced Dynamic Nuclear Polarization Complete Neglect of Differential Overlap cyclopentadienyl diethyl azodicarboxylate... 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term.7,76 Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical... 

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)... 

As we shall see, all relaxation rates are expressed as linear combinations of spectral densities. We shall retain the two relaxation mechanisms which are involved in the present study the dipolar interaction and the so-called chemical shift anisotropy (csa) which can be important for carbon-13 relaxation. We shall disregard all other mechanisms because it is very likely that they will not affect carbon-13 relaxation. Let us denote by 1 the inverse of Tt. Rt governs the recovery of the longitudinal component of polarization, Iz, and, of course, the usual nuclear magnetization which is simply the nuclear polarization times the gyromagnetic constant A. The relevant evolution equation is one of the famous Bloch equations,1 valid, in principle, for a single spin but which, in many cases, can be used as a first approximation. 

Using the OPENCORE spectrometer, a research group led by M. Kitagawa in Osaka have developed an experimental setup for dynamic nuclear polarization (DNP) using electron spins in the photo-excited triplet state. This nuclear hyperpolarization technique, called hereafter triplet DNP,... 

K. Takeda, Triplet State Dynamic Nuclear Polarization. VDM Verlag, 2009. 

A future optical device exploiting these two discoveries could write, read and operate on electron spins, while using patterned magnetic regions as memory elements. The region of large nuclear polarization rotates electron spins as they pass by- one necessary operation for quantum computing."... 

Griffin RG, Prisner TF (2010) High field dynamic nuclear polarization-the renaissance. Phys Chem Chem Phys 12 5737-5740... 

The majority of double-resonance solid-state NMR experiments involving spin-1/2 nuclei use transfer of nuclear polarization via dipolar cross polarization (CP) to enhance polarization of the diluted spins S with small gyromagnetic ratio ys and significant longitudinal relaxation time T at the expense of abundant spins I with large y, and short 7 [215]. Typically, CP is used in combination with MAS, to eliminate the line broadening due to CS A, as well as with heteronuclear decoupling. To achieve the / S CP transfer, a (n/2)y pulse is applied at the I spin frequency,... 

Finally, the use of DNP of shallow donors to enhance both 67Zn and surface ft nuclear polarizations has been demonstrated in ZnO nanoparticles by observation of EPR features rather than direct NMR observation [85, 87]. The electronic wavefunctions of these donors in ZnO have been probed by ENDOR experiments [36, 97], There is much potential for directly observing NMR with the sensitivity greatly enhanced by DNP not only in ZnO but in other nanoparticles as well. 

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